Director of National Intelligence Dan Coats added that the jihadists, who once held vast swaths of territory in Syria and Iraq but are now reduced to a shrinking enclave, would exploit any reduction in counter-terror pressure to stage a comeback.

President Donald Trump last month announced he was ordering a full withdrawal of the 2,000 US troops from Syria. Senior US officials have since given contradictory statements about US intentions, but the Pentagon said it had begun the withdrawal, although how long it would take remained uncertain.

"ISIS still commands thousands of fighters in Iraq and Syria, and it maintains eight branches, more than a dozen networks, and thousands of dispersed supporters around the world, despite significant leadership and territorial losses," Coats said in a new report to Congress, using an alternate name for the group.

"The group will exploit any reduction in CT pressure to strengthen its clandestine presence and accelerate rebuilding key capabilities, such as media production and external operations.

"ISIS very likely will continue to pursue external attacks from Iraq and Syria against regional and Western adversaries, including the United States."

IS proclaimed a "caliphate" in 2014 which at its peak reached the size of Britain.

But with support from a US-led military coalition, the Kurdish-led Syrian Democratic Forces are now in the final stages of an assault launched more than four months ago against the jihadists' last bastion: a four-square-kilometre pocket of territory in eastern Syria.

Coats' report said IS was focusing on exploiting sectarian tensions in Iraq and Syria, adding it "probably realizes that controlling new territory is not sustainable in the near term."

"We assess that ISIS will seek to exploit Sunni grievances, societal instability, and stretched security forces to regain territory in Iraq and Syria in the long term," he added.

On the subject of Al-Qaeda, the once mighty terror outfit responsible for the 9/11 attacks, the report said that while the group's leaders were encouraging attacks against the West including the US, most of its affiliates' "attacks to date have been small scale and limited to their regional areas."

It added Al-Qaeda's affiliates in East and North Africa, the Sahel, and Yemen "remain the largest and most capable terrorist groups in their regions.

"All have maintained a high pace of operations during the past year, despite setbacks in Yemen, and some have expanded their areas of influence."

Al-Qaeda elements in Syria were continuing to undermine efforts to resolve that conflict, while its South Asia branch was providing support to the Taliban.