More than 100 prisoners of the Islamic State extremist movement have escaped in Syria in the chaos since Turkey's incursion, a senior US official said on Wednesday.

"We would say the number is now over 100. We do not know where they are," James Jeffrey, the State Department pointman on Syria, told the House Foreign Affairs Committee when asked about the detainees.

Turkey launched a military operation in Syria after President Donald Trump agreed to pull US troops who were allied with the Syrian Democratic Forces, the Kurdish-led group that bore the brunt of the fight against the Islamic State group.

Jeffrey said the Kurdish fighters were still guarding prisoners from the extremist group, despite their warnings that they will need to devote resources to fighting Turkey instead.

"Almost all of the prisons that the SDF were guarding are still secured. The SDF still has people there," Jeffrey said.

"We are monitoring that as best we can. We still have people in Syria working with the SDF and one of those priorities is these prisons," he said.

The remarks nevertheless appeared at odds with Trump's claim earlier that "Kurds are safe and have worked very nicely with us. Captured ISIS prisoners secured."

The Kurdish fighters have pulled out of a key border area as part of a US-brokered agreement with Turkey to end the offensive.

Turkey links the Syrian Kurdish fighters to PKK separatists at home, who are considered a terrorist group by the United States and the European Union.

Allegations of war crimes

Jeffrey also charged that Turkey or Turkish-allied Syrian forces have committed war crimes, although he dismissed allegations by a lawmaker that President Recep Tayyip Erdogan was carrying out ethnic cleansing against the Kurds.

"We haven't seen any widespread ethnic cleansing in that area since the Turks have come in. Many people have fled because they were concerned about these Turkish-supported Syrian opposition forces, as are we," Jeffrey said.

"We have seen several incidents which we consider war crimes," he added.

He did not specify the incidents, but the United States earlier said it was appalled when pro-Turkish forces pulled 35-year-old Kurdish politician Hevrin Khalaf out of a car and killed her, with purported footage released on video.

Jeffrey said the United States was also investigating Kurdish allegations that Turkey used white phosphorus -- which can be used to create a smoke-screen but is also a deadly incendiary weapon and is banned under international law.

The envoy faced heated criticism from lawmakers who said that Trump strengthened the Islamic State group as well as Russia, whose forces are heading to northern Syria in an agreement with Erdogan in the wake of the US pullout.

"I think that it will affect us for years and years and decades to come and I think it will go down as one of the major American blunders in history," said Representative Eliot Engel, the Democrat who heads the House Foreign Affairs Committee.

"I just think what we've done there is shameful," he said.