Multiple US service members were killed when a suicide bomber blew himself up at a restaurant in the key city of Manbij in Syria's north, the deadliest attack against US forces since they first deployed in Syria in 2015.

Trump claimed on December 19 that the US and its allies had "beaten" IS, and he ordered an immediate troop withdrawal from the war-torn country.

While many Americans cheered Trump's withdrawal decision, fed up after years of costly and seemingly interminable conflicts, observers said Wednesday's attack shows he was too hasty.

Charles Lister, a senior fellow at the Middle East Institute, said the attack demonstrated how IS can now conduct a low-level insurgency in Syria.

This is "precisely how this jihadist organization has adapted and gone back on the attack in years past," Lister said.

"Trump's order was reckless and driven far more by domestic political concerns than it was by facts on the ground."

To bolster his argument that IS had been defeated, Trump highlighted how much territory the group had lost since it declared a "caliphate" across swaths of Syria and Iraq in 2014.

But the jihadists still hold small pockets of land in the Euphrates River Valley, and thousands of fighters are thought to remain in Syria.

"To suggest ISIS is 'defeated' because it no longer controls territory is to fundamentally misunderstand how ISIS and similar organizations seek to operate," Lister said.

- 'No time to retreat' -

The American president did not immediately react to the US troop deaths, but Vice President Mike Pence double-downed on Trump's declaration that the US would pull all of its 2,000 troops from Syria.

"The caliphate has crumbled and ISIS has been defeated," Pence said, without referencing the attack.

The White House referred questions back to the Pentagon.

Members of Trump's own Republican Party have spoken out against his withdrawal plans, including Senator Lindsey Graham, usually a stalwart supporter.

"My concern about the statements made by President Trump is that (he) had set in motion an enthusiasm by the enemy we are fighting," Graham said.

"I would hope the president would look long and hard with where he's headed in Syria. I know people are frustrated, but we are never going to be safe here unless we are willing to help people over there who will stand up against this radical ideology."

Senator Marco Rubio, another Republican, said IS's claim for the attack serves as a "tragic reminder that ISIS not been defeated and is transforming into a dangerous insurgency."

"This is no time to retreat from the fight against ISIS. Will only embolden & strengthen them," Rubio said on Twitter.

In the weeks since he gave the withdrawal order, and the Pentagon began to implement it, Trump and members of his administration have delivered mixed messages about the timeline.

National Security Advisor John Bolton last week announced conditions for a withdrawal that appeared to slow it for an undetermined amount of time.

Adding to the confusion, the Pentagon then said the US has already begun "the process of our deliberate withdrawal" from Syria, but stressed this was only for "non-essential" military gear -- and not troops.