The US envoy to the global coalition fighting Islamic State has resigned in protest over Donald Trump’s abrupt decision to withdraw US troops from Syria.

Brett McGurk announced his departure just two days after the president’s defence secretary, Jim Mattis, decided to quit.

McGurk had said it would be “reckless” to consider IS defeated and therefore would be unwise to bring American forces home.

His resignation letter was submitted on Friday to Secretary of State Mike Pompeo.

McGurk, appointed to the post by President Barack Obama in 2015 and retained by Trump, said in his resignation letter that the militants were on the run, but not yet defeated, and that the premature pullout of American forces from Syria would create the conditions that gave rise to IS.

He originally planned to leave the job in mid-February after a US-hosted meeting of foreign ministers from the coalition countries, but he felt he could continue no longer after Trump’s decision to withdraw from Syria and Mattis’ resignation.

The president continued to insist on Saturday that IS had been defeated.

In a letter published by the Pentagon on Thursday, Mattis emphasised the importance of standing up for US allies in an implicit criticism of the president’s actions.

He wrote: “While the US remains the indispensable nation in the free world, we cannot protect our interests or serve that role effectively without maintaining strong alliances and showing respect to those allies.

“Because you have a right to a Secretary of Defense whose views are better aligned with yours on these and other subjects, I believe it is right for me to step down from my position.”

President Trump is acting to pull all 2,000 US troops from Syria and has now declared victory over IS, contradicting his own experts’ assessments. Many have called his action rash and dangerous.

McGurk’s deputy, retired Lt Gen Terry Wolff, who served three tours of active duty in Iraq, will take over from him initially.