Lindsey Graham, who had earlier called the withdrawal decision a “huge Obama-like mistake”, told journalists: “The president assured me he is going to make sure he gets the job done.
“He promised to destroy Isis. He’s going to keep that promise.
“I think we’re slowing things down in a smart way,” the South Carolina senator said.
Implying that Mr Trump might rethink or delay the withdrawal of US troops, Mr Graham said: “I think we’re in a pause situation where we are re-evaluating what’s the best way to achieve the president’s objective of having people pay more and do more.”
Speaking later on CNN, Mr Graham also expressed concern that the US withdrawal would leave “our allies the Kurds” in the north of Syria exposed to attacks from Turkey.
“If we leave now, the Kurds are going to get slaughtered,” he said. “The president is reconsidering how we do this. He’s frustrated, I get that.”
What is the US presence in Syria?
US ground troops first became involved in Syria in autumn 2015 when then-President Barack Obama sent in a small number of special forces to train and advise local Kurdish fighters who were fighting IS.
The US did this reluctantly after several attempts at arming anti-IS groups had descended into chaos.
Over the intervening years the numbers of US troops in Syria increased, standing today at some 2,000, though some estimates place the number perhaps even higher.
A network of bases and airstrips has been established in an arc across the north-eastern part of the country.
The US has also been part of an international coalition conducting air strikes against IS and other militants.