Donald Trump has unleashed a flurry of defensive tweets about his decision to pull US forces out of Syria after special envoy Brett McGurk followed defence secretary Jim Mattis in resigning in protest.

Mr McGurk, the US envoy to the global coalition fighting ISIS, became the latest experienced national security figure to quit over the troop withdrawal on Saturday.

Mr Trump played down the development, tweeting that he did “not know” the special envoy and calling his resignation a “nothing event”. He noted Mr McGurk planned to leave soon anyway and added: “Grandstander?”

The president also described his relationship with Mr Mattis – who quit on Thursday – as “interesting”. He insisted his defence secretary had been given “the resources” he had asked for. The outgoing defence secretary had opposed withdrawing troops from Syria as well as Mr Trump’s plan to reduce the US presence in Afghanistan.

Mr Trump tweeted: “When President Obama ingloriously fired Jim Mattis, I gave him a second chance. Some thought I shouldn’t, I thought I should. Interesting relationship - but I also gave all of the resources that he never really had. Allies are very important - but not when they take advantage of US.”

He added: “If anybody but your favourite President, Donald J. Trump, announced that, after decimating ISIS in Syria, we were going to bring our troops back home (happy & healthy), that person would be the most popular hero in America.”

The president is facing a barrage of criticism from senior Republicans over the withdrawal. Senator Marco Rubio called it a “colossal mistake,” while Senate majority leader Mitch McConnell said he was “distressed” at Mr Mattis’ departure from government and suggested the chaos over US foreign policy was “putting America in danger”.

Appointed as an anti-Isis envoy by President Barack Obama in 2015 and retained by Mr Trump, Mr McGurk said the president’s Syria announcement had been a “shock” and “complete reversal of policy”.

In his resignation letter, the special envoy argued that Isis were not yet completely defeated in the wartorn country and a pull-out of US forces was premature.

Brett McGurk, special envoy for the global coalition to defeat ISIS, during a visit to Syria (Reuters)

Mr Trump’s move “left our coalition partners confused and our fighting partners bewildered with no plan in place or even considered thought as to consequences,” he said.

Shortly after news of the latest resignation broke, Mr Trump again defended his decision to pull all of the roughly 2,000 US forces from Syria in the coming weeks.

“We were originally going to be there for three months, and that was seven years ago - we never left,” Mr Trump tweeted.

“When I became President, ISIS was going wild. Now ISIS is largely defeated and other local countries, including Turkey, should be able to easily take care of whatever remains. We’re coming home!”

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Although the civil war in Syria has gone on since 2011, the US did not begin launching air strikes against IS until September 2014, and American troops did not go into Syria until 2015.

Mr Trump has found some support for his decision, however. Ron Paul, the former Libertarian Party presidential candidate, said it was “fantastic” move.