Donald Trump's special envoy for the Syrian conflict said the administration made clear "at every level" that it opposed Turkey's invasion of northern Syria and that the resulting offensive has undermined the fight against Islamic State.

"Turkey launched this operation despite our objections, undermining the D-ISIS campaign, risking endangering and displacing civilians, destroying critical civilian infrastructure, and threatening the security of the area," Ambassador James Jeffrey told the Senate Foreign Relations Committee on Tuesday, using an acronym for the coalition to defeat the Islamic State. "Turkey's military actions have precipitated a humanitarian crisis and set conditions for possible war crimes."

Despite the setback, Jeffrey said the U.S. will work with Turkey and "local partners" to continue the fight against Islamic State. Lawmakers of both parties have warned that Trump's abrupt decision to begin withdrawing about 1,000 U.S. troops gave Turkey a green light to invade, which will prove disastrous for regional stability and deadly for U.S.-backed Kurdish fighters that fought against Islamic State.