The U.S. special presidential envoy for the global coalition to defeat Daesh has submitted his resignation, CBS News reported Saturday.

Brett McGurk's resignation will take effect Dec. 31. He has served the post for three years.

McGurk worked under the past three administrations, becoming one of the few persons appointed by former President Barack Obama to keep his post under U.S. President Donald Trump. He also served under the Bush administration.

The envoy had originally planned to leave his post in 2019 but decided to accelerate his resignation over strong disagreements with Trump over the withdrawal of U.S. troops from Syria, the news channel reported quoting anonymous sources.

The decision to remove 2,000 American troops out of Syria came from a telephone call with Turkish President Recep Tayyip Erdogan, according to The Associated Press, and Trump made the decision without consulting his top national security officials or any of the U.S.'s key allies.

McGurk's resignation comes as a surprise because the envoy had told reporters at the State Department that the U.S. was going to remain committed to the fight against Daesh terrorist forces last week.

"Nobody is declaring a mission accomplished. Defeating a physical caliphate is one phase of a much longer-term campaign," McGurk told reporters.

McGurk sent in his resignation on Friday, a day after Secretary of Defense James Mattis quit saying his views were not aligned with the president. Mattis will end his post in February.

Mattis noted that the most important part of the U.S.’s defense strategy is maintaining alliances with other nations, while Trump has been calling for increasing isolationism during his presidency as part of his “America First” agenda.

After leaving his post with the Trump administration, McGurk plans to take a position at Stanford University, according to CBS News.