U.S. President Donald Trump's special envoy for Syria said on Wednesday that U.S. forces had seen evidence of war crimes by Turkish forces in Syria, during their recent offensive against Kurds.

"We haven't seen widespread evidence of ethnic cleansing" by Turkey, but there had been reports of "several incidents of what we consider war crimes," said James Jeffrey, special representative for Syria, at a House of Representatives hearing.

He said U.S. officials were looking into those reports and had demanded an explanation from Turkey's government. He also said U.S. officials were investigating a report that Turkey had used restricted burning white phosphorus during its offensive.

Trump spoke Wednesday from the White House announcing a lasting cease-fire in northeastern Syria and thanked Turkish President Recep Tayyip Erdogan. Trump added that while some Islamic State fighters escaped most had been recaptured, a direct contradiction to what Jefferies had told the U.S. Senate on Tuesday.

Jeffrey and Matthew Palmer, a deputy assistant secretary of state who handles issues including relations with Turkey, spent a second straight day testifying in the U.S. Congress.

Jeffrey confirmed Secretary of Defense Mark Esper's statement that more than 100 Islamic State fighters had escaped. "We do not know where they are," Jeffrey told the hearing.

Jeffrey had said in Senate testimony on Tuesday that "dozens" of the militants had escaped prison since the Turkish offensive in northeastern Syria began about two weeks ago.

Many lawmakers, including Trump's fellow Republicans as well as Democrats, disagree with Trump's decision to withdraw troops from Syria, which cleared the way for Turkish troops to cross the border to fight against Kurdish forces who for years helped U.S. troops fight against Islamic State militants.

"How could the United States do something so senseless, so disgraceful, so contrary to our values?" Democratic Representative Eliot Engel, chairman of the House Foreign Affairs Committee, said as he opened the hearing.