Boris Johnson has urged President Recep Tayyip Erdogan to halt his military assault on Kurdish-held northern Syria.
In a telephone call to the Turkish leader, the Prime Minister voiced his 'grave concern' that the action could worsen the humanitarian situation in the region and undermine the fight against the so-called Islamic State (ISIS).
He called on him to enter into dialogue with a view to reaching agreement on a ceasefire.
'He expressed the UK's grave concern about Turkey's military operation in northern Syria which he said could further worsen the humanitarian situation there and undermine the progress made against Daesh (ISIS),' a No 10 spokesman said.
'The Prime Minister underlined that Turkey is an important partner for the UK and a Nato ally.
'He recognised Turkey's role at the forefront of the fight against Daesh and its generosity in supporting refugees who've fled the civil war in Syria.
'But the Prime Minister was clear that the UK cannot support Turkey's military action.
'He urged the president to end the operation and enter into dialogue, and said the UK and international partners stand ready to support negotiations towards a ceasefire.'
Johnson's concerns come after 200,000 people were displaced in a military assault in northern Syria as Turkish forces claimed on Saturday they have seized control of a key border town.
Turkey's offensive, triggered by President Trump's decision to withdraw troops from the area, has allowed five Islamic State prisoners to escape Kurdish captivity as security services begin to collapse under the weight of the onslaught.
Turkey-backed Syrian troops were said to have taken over Ras al-Ain this morning as Turkey's offensive against a Kurdish militia in the region entered its fourth day.
'The [Syrian rebel] national army took control of the town centre this morning. Inspections are being conducted in residential areas,' a senior Turkish official said.
Kurdish authorities have, however, denied this is the case. 'Ras al-Ain is still resisting and clashes are ongoing,' the Kurdish-led Syrian Democratic Forces said.
The Syrian Observatory for Human Rights also reported that the town, a major target of the Turkish offensive, had yet to be completely taken.
But Turkey's Ministry for Defence tweed to say: 'Ras al-Ain's residential centre has been taken under control through the successful operations in the east of Euphrates river'.
It marked the biggest gain made by Turkey since the invasion began Wednesday.
It comes as Turkish troops stepped up their bombardment in northeastern Syria on Saturday as the death toll rose to 74 Kurdish YPG fighters and 30 civilians.
It is also believed nearly 200,000 people have been displaced as a result of the offensive, the Syrian Kurdish-led administration said.
On the front lines, thick plumes of smoke rose around Ras al-Ain, one of two Syrian border areas targeted in the offensive as Turkey-backed forces ramped up their military assault.
Intense gunfire also resounded from within the town itself, while warplanes could be heard flying overhead, reports said.
The continued push by Turkey into Syria comes days after US President Donald Trump cleared the way for Turkey's air and ground offensive, pulling back US forces from the area and saying he wanted to stop getting involved with 'endless wars'.
A Syrian Democratic Forces official inside Ras al-Ain said fighters had pushed back Ankara's forces but clashes were ongoing.
It was quieter at Tel Abyad, the operation's other main target some 75 miles to the west, with only occasional shell fire heard in the area.
In the countryside, Kurdish fighters have been losing ground. Turkish forces overran 18 villages overnight, most of them near Tal Abyad, raising the number they have taken so far to 23.
Aid agencies have warned of a looming humanitarian crisis for up to 1.6million people as Kurdish families jammed roads heading south to the fringes of the 20-mile border zone that Erdogan has said he will occupy.
The intensity of the fighting is preventing aid workers from providing vital services. Last night, a spokesman for 15 charities in the region said: 'Once again, families and children find themselves caught up in deadly violence.
'Urgent action is needed to ensure that the humanitarian situation in north-east Syria does not worsen further.
'An estimated 450,000 people live within three miles of the Syria-Turkey border and are at risk if all sides do not exercise restraint and prioritise the protection of civilians.'
The epicentre of the crisis was Ras al-Ain, expected to be the first town to fall to Ankara’s forces. Columns of exhausted residents filed into the nearby town of Tal Tamr where aid workers handed out food, blankets and water.
Tanks and heavy artillery continued to pour across Turkey’s border with Syria at Sanliurfa to bolster the assault on Kurdish-held border towns. A dense cloud of black smoke billowed from Ras al-Ain following an intense ground and air assault yesterday.
Turkish forces and Syrian rebels were pushing deeper into Kurdish-occupied northern Syria. Turkish forces also captured 18 villages in the area as part of an operation codenamed Peace Spring. Five IS prisoners broke out of prison in Qamishli during a riot prompted by a Turkish air strike.
According to the latest figures, 74 Kurdish YPG fighters have died in clashes. Turkey has said it aims to defeat the YPG, which it sees as an enemy for its links to PKK militants who have fought a decades-old insurgency in Turkey in which 40,000 people have been killed.
Mr Trump has come under fire from senior members of his own party for ‘abandoning’ Kurdish fighters who fought alongside the US and the UK to defeat Islamic State in Syria and Iraq.
Concerns were also raised over a decision to permit Turkish military personnel to take part in a key Nato training mission off the western coast of Scotland.
The Ministry of Defence said last night: ‘Turkey is a Nato member and has therefore contributed two liaison officers to the Nato exercise Joint Warrior, who are taking part alongside personnel from 13 other countries.’
On Friday evening, Turkish President Tayyip Erdoğan dismissed mounting international criticism of the operation and said Turkey 'will not stop it, no matter what anyone says'.
The movement comes as the death toll among Syrian Kurdish-led fighters rose to 74, most of whom have been killed in the Tel Abyad area, the Syrian Observatory for Human Rights confirmed.
Observatory Director Rami Abdulrahman also said 49 fighters with Turkish backed Syrian rebel groups had been killed since the assault began on Wednesday.
The death toll among civilians in Syria had climbed to 30 on Saturday.
Turkey's Defense Ministry said 415 YPG militants had been 'neutralised' since the operation began, a term that commonly means killed.
There has been fierce international criticism of the assault and concern about its humanitarian consequences. The Syrian Kurdish-led administration said nearly 200,000 people have been displaced as a result of the offensive.
Youssef Hammoud, spokesman for the Turkey-backed National Army, added on Saturday that fighters had cut the 712 road that links Tel Abyad and Ras al Ain.
'This advance was on a new and surprise front between Tel Abyad and Ras al Ain. On this front, they were able to cut the roads linking together Suluk, Tel Abyad, Ras al-Ain with the villages in the area,' he said.
The United States has ramped up its efforts to persuade Ankara to halt the incursion against US-backed Kurdish forces, saying Ankara was causing 'great harm' to ties and could face sanctions.
Turkey opened its offensive after US President Donald Trump spoke by phone on Sunday with Erdogan and withdrew US troops who had been fighting alongside Kurdish forces.
Trump's decision drew swift bipartisan criticism that he was endangering regional stability and risking the lives of Syrian Kurdish allies who brought down the Islamic State group in Syria.
The Kurdish-led Syrian Democratic Forces were the main US ally in the fight against IS and lost 11,000 fighters in the nearly five-year battle against the extremists.
Turkish troops and allied Syrian opposition fighters have made gains recently capturing several northern villages in fighting and bombardment that left dozens of people killed or wounded.
Earlier, the Pentagon said US troops came under artillery fire from Turkish positions on Friday but none of its soldiers were wounded, near Syria's Kobani, some 37 miles west of the main area of conflict.
Turkey's Defense Ministry said its forces did not open fire at the US base and took all precautions to prevent any harm to it while it was responding to fire from a nearby area by the Kurdish YPG militia, which Ankara regards as a terrorist group.
'US and coalition soldiers were definitely not hit. Indeed the necessary coordination is being carried out by our headquarters and the Americans,' Turkish Defence Minister Hulusi Akar said on Saturday.
The Syrian Democratic Forces, with the Kurdish YPG as its main fighting element, now holds most of the territory that once made up Islamic State's 'caliphate' in Syria, and has been keeping thousands of fighters from the jihadist group in jail and tens of thousands of their family members in camps.
The Kurdish militia said the Turkish assault could allow the jihadist group to re-emerge as some of its followers were escaping from prisons.
In its first big attack since the assault began on Tuesday, Islamic State claimed responsibility for a deadly car bomb in Qamishli, the largest city in the Kurdish-held area, even as the city came under Turkish shelling.
Five Islamic State fighters fled a jail there, and foreign women from the group being held in a camp torched tents and attacked guards with sticks and stones, the SDF said.
US Defense Secretary Mark Esper has told his Turkish counterpart they should de-escalate the situation before it becomes 'irreparable', while European Council President Donald Tusk warned it could lead to a 'humanitarian catastrophe'.
Erdogan's spokesman, Ibrahim Kalin, responded to the criticism on Saturday, saying 'Turkey's fight is against terrorists, not Kurds or civilians'.
'Blackmail and threats will never deter Turkey from its just cause,' Kalin wrote on Twitter. 'God willing victory will be ours.'
US lawmakers introduced more legislation on Friday seeking stiff sanctions on Turkey over the offensive, underscoring unhappiness from both Democrats and President Trump's fellow Republicans in Congress over his Syria policy.
Ras al-Ayn is one of the biggest towns along the border and it is in the middle of the area that Turkey plans to set up its safe zone.
The ethnically and religiously mixed town with a population of Arabs, Kurds, Armenians and Syriac Christians had been under the control of Kurdish fighters since 2013.
IS members tried to enter Ras al-Ayn following their rise in Syria and Iraq in 2014 but failed.