Top Pentagon officials have denied the US is deserting its Kurdish allies in the face of a Turkish military offensive - although the future of a counterterrorism partnership with the Kurds was in grave doubt.
"We have not abandoned the Kurds. Let me be clear about that," Defence Secretary Mark Esper told reporters.
"We have not abandoned them. Nobody green-lighted this operation by Turkey - just the opposite.
"We pushed back very hard at all levels for the Turks not to commence this operation," he said.
He spoke shortly before American forces came under Turkish artillery fire at a small outpost in north-eastern Syria.
No Americans were hurt, and the Turks insist the Americans were not targeted. But the strike shows how swiftly the conflict is descending into turmoil.
Meanwhile, the humanitarian crisis sparked by the fighting is already beginning to bite, with tens of thousands of people displaced.
ITV News Correspondent Emma Murphy reports on the border between Turkey and Syria
The UN said 100,000 people had fled the violence as the number of casualties rose, while aid agencies warned nearly half a million people were at risk in north-eastern Syria.
Turkey launched its offensive in northern Syria just hours after US President Donald Trump pulled US troops from the area.
Many criticised the move by Mr Trump as the Kurds had been at the forefront on taking on the forces of ISIS.
Turkey considers the Syrian Kurdish fighters to be terrorists linked to a Kurdish insurgency inside Turkey and says the offensive is a counter-terrorism operation necessary for its own national security.
Despite the criticism, Turkish President Recep Tayyip Erdogan said his country “will not take a step back” from its offensive.
“We will never stop this step. We will not stop no matter what anyone says,” he said in a speech on Friday.
Turkish forces moved to seize control of key highways in northeastern Syria on Saturday.
Its state-run Anadolu news agency said that Turkey-backed Syrian opposition forces had taken control of the M-4 highway that connects the towns of Manbij and Qamishli.
Turkish troops also cut the route linking the northeastern city of Hassakeh with Aleppo, Syria's largest city and once commercial center, according to the Syrian Observatory for Human Rights, an opposition war monitor.
The Turkish incursion has complicated US military efforts in the region, even as Washington seeks to deter Iran from further attacks on Saudi Arabia following a drone and cruise missile assault in September that damaged key Saudi oil facilities.
Esper announced that he was sending dozens more fighter jets and additional air defences to Saudi Arabia, beefing up efforts to defend against Iran.
At the White House, Treasury Secretary Steven Mnuchin put Turkey on notice that it could face "powerful sanctions" for its military incursion, and that the US will "shut down the Turkish economy" if Ankara goes too far.
Mnuchin said the US hopes it will not have to use new, expanded sanctions authority that Trump has authorised.
The Turkish invasion also has raised the prospect of losing control of thousands of captured Islamic State fighters who are in detention facilities under the Kurds' control.
Esper called on the Turks to halt their offensive, but he told reporters that he has no indication they will.
He lamented "the dramatic harm" done to the two nations' relationship.