A top U.S. general warned on Tuesday that Islamic State would pose an enduring threat following a planned U.S. withdrawal from Syria, saying the militant group retained leaders, fighters, facilitators and resources that will fuel a menacing insurgency.

The remarks by U.S. General Joseph Votel, head of the U.S. military's Central Command, represent the latest warning by current and former U.S. officials about the risk of a resurgence by Islamic State following a planned U.S. withdrawal from Syria ordered in December by President Donald Trump.

"We do have to keep pressure on this network. ... They have the ability of coming back together if we don't," Votel told a Senate hearing.

He added that territory under Islamic State's control had been reduced to less than 20 square miles (5,180 hectares) and would be recaptured by U.S.-backed forces prior to the U.S. withdrawal, which he said would be carried out in a "deliberate and coordinated manner."

Votel told the Senate hearing he was not consulted ahead of Trump's surprise decision to withdraw America's some 2,000 troops from Syria, which helped trigger the resignation of his defense secretary, Jim Mattis.

Trump's Syria withdrawal has fueled rare, vocal opposition from within his own Republican party.

The Republican-led U.S. Senate on Monday backed largely symbolic legislation that broke with Trump by opposing plans for any abrupt withdrawal of troops from Syria and Afghanistan.

It warned "a precipitous withdrawal" could destabilize the region and create a vacuum that could be filled by Iran or Russia.

The Pentagon's own internal watchdog released a report on Monday saying Islamic State remained an active insurgent group and was regenerating functions and capabilities more quickly in Iraq than Syria.

"Absent sustained (counterterrorism) pressure, ISIS could likely resurge in Syria within six to twelve months and regain limited territory," the report from the Pentagon's Inspector General said.