Pressure is building in the US to stop Turkey continuing its offensive against Kurdish-held areas in Syria.
Defence Secretary Mark Esper warned of "serious consequences" while Treasury Secretary Steven Mnuchin held out the prospect of fresh sanctions.
President Donald Trump's decision to withdraw US troops effectively triggered the Turkish incursion.
He has said he would like the US to negotiate a truce between Nato ally Turkey and the Kurds.
Turkish President Recep Tayyip Erdogan, however, says the military operation will continue. Kurdish leaders accuse the US of stabbing them in the back.
Meanwhile, the Pentagon said US forces near the town of Kobane – which were not included in the US withdrawal – had come under Turkish artillery fire on Friday evening.
According to the UN more than 100,000 people have fled their homes since the offensive began on Wednesday.
President Erdogan says he wants to create a "safe zone" in northern Syria free of Kurdish militias which could also be home to Syrian refugees.
One major concern for the international community is the fate of thousands of suspected IS prisoners, including many foreign nationals, being guarded by Kurdish-led forces in the region.
What happened to the US troops?
On Friday, the Pentagon said its base near the northern Syrian town of Kobane – an area known by Turkey to have US forces present – had seen shell fire from Turkish positions.
"All US troops are accounted for with no injuries," Navy Captain Brook DeWalt said in a statement. "The US demands that Turkey avoid actions that could result in immediate defensive action."
Turkey denied deliberately targeting US forces./BBC news