"Our work over the coming year will require intensified effort to stabilize communities liberated from Daesh," McGurk said in the letter sent to coalition members on Friday.
The envoy stressed that the United States will keep its presence in Syria as the fighting against the group is still ongoing.
"The United States is prepared to remain in Syria until we are certain that Daesh is defeated," McGurk said further.
The United States has been leading a coalition of more than 70 nations aimed at defeating Daesh operating in Iraq and Syria since 2014. The activities of the coalition in Syria are authorized neither by the UN Security Council nor Damascus.
On December 11, Russian President Vladimir Putin announced the withdrawal of main part of Russian troops in Syria, which have been present in the country since 2015 under the request of Syrian President Bashar Assad, as Daesh has been defeated in Syria. US President Donald Trump said later that 100 percent of Syrian and Iraqi territory had been liberated from the terrorist group, and US still keeps its presence.