A top US official in the fight against the Islamic State (IS) militant group has quit over President Donald Trump's decision to pull troops from Syria, reports say.
Brett McGurk, the US special presidential envoy for the Global Coalition to Defeat IS, brought his departure forward from February.
Before Trump's announcement he had insisted that the US would continue working against IS in Syria. It follows the resignation of Defence Secretary Jim Mattis on Thursday.
Mattis had also opposed withdrawing troops from Syria as well as reducing the US presence in Afghanistan.
McGurk is an experienced diplomat who was appointed to his current role in 2015 under the Obama administration.
McGurk said in his resignation letter that IS militants in Syria were on the run but not yet defeated. He said that withdrawing US forces from Syria would create the conditions that gave rise to IS, the BBC reported.
In early December he told reporters: "We want to stay on the ground and make sure that stability can be maintained in these areas."
He went on to say: "It would be reckless if we were just to say, well, the physical caliphate is defeated, so we can just leave now. I think anyone who's looked at a conflict like this would agree with that."
In an email to staff quoted by the New York Times, he said Trump's decision to pull out troops "came as a shock and was a complete reversal of policy". It "left our coalition partners confused and our fighting partners bewildered", he said.
"I ultimately concluded I could not carry out these new instructions and maintain my integrity," he went on to say.
Trump announced his decision to withdraw some 2,000 US troops from Syria on Wednesday, asserting that IS had been defeated.
He has not yet reacted to McGurk's resignation.
But on Saturday he continued to insist that the decision to pull out was the right one and that, now that IS was defeated on the ground, other players could take care of the situation.
However, important allies including senior Republicans and foreign powers have disputed the claim and say the move could lead to a resurgence of IS.
A Kurdish-led alliance, the Syria Democratic Forces (SDF) has also warned that IS could recover.
US troops have helped rid much of Syria's north-east of the jihadist group, but pockets of fighters remain.
A recent US report said there were still as many as 14,000 IS militants in Syria and even more in neighbouring Iraq.