President Donald Trump in December declared victory over IS, saying the US had "beaten them badly" and "we have won" as he announced the 2,000 or so American troops -- most of them special forces commandos -- would withdraw from the country.

General Raymond Thomas, who heads US Special Operations Command, was asked at a hearing in the US Senate what winning in Syria would look like.

"I'd be hesitant to use the term winning, as opposed to the objective," Thomas told the Armed Services Committee.

That "objective is to reduce the threat in that area and to be able to maintain persistent capabilities so that an external threat cannot emanate from that in the future," he added.

Republican Senator Josh Hawley asked Thomas if he was satisfied the US was at that point.

"I do not think we're there yet," Thomas said.

"We're on the verge of diminishing the threat and then in the process of determining what the residual capability needs to remain in place in the region to ensure that we are securing that, securing that objective."

Since Trump's announcement, US officials have dialed back talk of a clean victory over IS, stressing the group will remain indefinitely as an insurgency after they have lost control of the area they once claimed as a "caliphate."

Thomas went on to say that he was unsure what the "end state" in Syria might look like.

"This has been one of the most complex challenges that our force has dealt with in quite a while," he told lawmakers.

"I am concerned that we are not overly restricted in the end state, and I don't honestly know what that is."

General Joseph Votel, who heads US Central Command, this month said Trump did not consult him ahead of the decision to pull US troops from Syria.

A primary criticism of Trump's withdrawal, aside from its rushed nature, is that it leaves US-backed Kurdish fighters vulnerable to attack from Turkey.

No plan to protect the Kurds in the Syrian Democratic Forces has been announced, but Thomas said: "We are working on solutions to maintain some contact and some level of support for them."

In Syria, IS jihadists on Thursday were mounting a desperate defense of their last speck of terrain, in the small town of Baghouz.