The US military commander overseeing American troops in the Middle East has told a Senate hearing that he was not consulted ahead of Donald Trump’s surprise decision in December to pull US troops out of Syria.

“I was not consulted,” said Gen Joseph Votel, head of the US military’s Central Command, who warned that Islamic State will continue to pose a threat following a US withdrawal.

Votel said on Tuesday that the militant group retained leaders, fighters, facilitators and resources that will continue to fuel a menacing insurgency.

The remarks represent the latest warning by current and former US officials about the risk of an Isis resurgence after US troops are withdrawn.

“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 Isis control had been reduced to less than 20 sq miles (5,180 hectares) and would be recaptured by US-backed forces prior to the US 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 about 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 US 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 Isis remained an active insurgent group and was regenerating functions and capabilities more quickly in Iraq than Syria.

“Absent sustained [counter-terrorism] pressure, Isis could likely resurge in Syria within six to 12 months and regain limited territory,” the report from the Pentagon’s inspector general said.