A senior Republican U.S. senator said he emerged from a White House meeting with President Donald Trump on Sunday reassured that Trump is committed to defeating Islamic State even as he plans to withdraw American troops from Syria.

Senator Lindsey Graham had warned that removing all 2,000 U.S. troops from Syria would hurt national security by allowing Islamic State to rebuild, betraying U.S.-backed Kurdish fighters of the YPG militia battling remnants of the militant group, and enhancing Iran's ability to threaten Israel.

During a morning television interview, Graham said he would ask Trump to slow down the troop withdrawal, which was announced earlier this month and drew widespread criticism.

An ally of Trump, although he has opposed some of his foreign policy decisions, Graham was more upbeat after the meeting.

"We talked about Syria. He told me some things I didn't know that made me feel a lot better about where we're headed in Syria," Graham, an influential voice on national security policy who sits on the Senate Armed Services Committee, told reporters at the White House.

"We still have some differences but I will tell you that the president is thinking long and hard about Syria - how to withdraw our forces but at the same time achieve our national security interests," Graham said.

Asked if Trump had agreed to any slowing down of the troop withdrawal, Graham said: "I think the president's very committed to making sure that when we leave Syria, that ISIS is completely defeated."

He said Trump's trip to Iraq last week was an eye-opener and he understood the need to "finish the job" with Islamic State, also known as ISIS.

"I think the president has come up with a plan with his generals that makes sense to me," Graham said.

Graham said later on Twitter that Trump would make sure that any withdrawal from Syria "will be done in a fashion to ensure: 1)ISIS is permanently destroyed 2)Iran doesn't fill in the back end. And 3)our Kurdish allies are protected."

The Pentagon says it is considering plans for a "deliberate and controlled withdrawal." One option, according to a person familiar with the discussions, is for a 120-day pullout period.