The Iraqi military has thrown American plans into chaos by saying that U.S. troops withdrawing from Syria do not have permission to stay in Iraq.
U.S. forces have permission from the Kurdish regional government to transit through Iraq but have no approval to stay in the country, a statement said today.
It contradicts claims by U.S. Defense Secretary Mark Esper, who said troops leaving Syria would go to western Iraq and continue to conduct operations against ISIS.
Esper said he had spoken to his Iraqi counterpart about shifting more than 700 troops into Iraq and 'help defend' the country but those plans now appear to be in disarray.
American forces based in Iraq could cross into Syria to conduct operations against ISIS militants, the Pentagon has suggested.
President Donald Trump ordered troops out of northern Syria on October 7, opening the door for Turkey's offensive against Kurdish fighters there.
Trump has promised to bring troops home from 'endless wars' in the Middle East but has been condemned for abandoning the Kurds who helped defeat ISIS in Syria.
Dozens of U.S. armoured vehicles have already crossed into Iraqi Kurdistan from Syria with American soldiers aboard.
U.S. forces have withdrawn from several bases in Syria, including from the key town of Manbij and another close to Kobane close to the Turkish border.
There is a U.S. base in the autonomous region and a withdrawing convoy crossed the Tigris River at a border post on Monday.
Angry Kurds have blocked U.S. troops in the streets as they cross from Syria into Iraq after Washington pulled the plug on their support.
American troops were even pelted with potatoes as they passed through a Syrian town on their way to Iraq on Monday.
Footage posted to Twitter showed a vehicle bearing the American flag struggling to make its way forward as several locals stand in front of it.
The Pentagon is now considering keeping a small U.S. force in north-eastern Syria to protect oilfields.
The United States currently has 5,200 troops posted in Iraq, deployed as part of a Washington-led coalition against the ISIS jihadists.
The U.S. presence at several bases across Iraq is already controversial, with numerous political groups and pro-Iran Shiite armed groups demanding their expulsion.
The Kurdish fighters in Syria are dominated by the YPG, a militia which Turkey regards as a terrorist group.
Since the Turkish offensive began on October 9, at least 114 civilians have been killed and some 300,000 people have been displaced.
Mr Erdogan - who has previously hinted at nuclear ambitions - wants a buffer zone against the Kurds along the Turkish border.
Washington brokered a five-day ceasefire last week to allow the besieged Kurdish fighters, but it expires today.