The group "certainly still remain a threat," he said. "They have the potential to resurge if we take pressure off of them for too long."

The general said he did not see the threat of an immediate IS comeback.

"But the more time we take pressure off of them, the more of that threat will continue to grow," he said.

At a Pentagon press conference, he said the structural weakness of IS is shown by their failure to take advantage of demonstrations in Iraq calling for political reforms since October.

More than 460 protesters have been killed, and demonstrators are angry that few Iraqi security personnel have been charged for the violence.

The allies at the heart of the international coalition have over the last few months been evaluating the position of the jihadist movement whose self-declared "caliphate" once spanned parts of Iraq and Syria. It collapsed last March after years of battle with coalition-backed forces.

IS went underground and reverted to well-honed guerrilla tactics that continued to do damage.

The coalition wanted to determine whether the group is "executing some sort of strategic patience, waiting for an opportunity that they can exploit, or are they truly on the ropes a bit more and lacking in capability and capacity?" Grynkewich said.

He said the Iraqi protests helped the coalition to refine its assessment "that it's actually ISIS is a little bit more on the lack of capability and capacity side, than strategically patient," using another acronym for the jihadists.

Tensions between Washington and Tehran boiled over onto Iraqi soil this month. The US killed top Iranian general Qasem Soleimani in Baghdad and Tehran retaliated against an Iraqi base hosting American soldiers, some of whom were hurt.

Furious at the US hit, Iraq's parliament voted January 5 to oust all foreign troops, including about 5,200 American soldiers deployed alongside local forces.

Coalition troops have ostensibly reduced their operations in Iraq since then, even if cooperation with the Iraqi army continues discreetly, according to several US military sources.

US President Donald Trump and his Iraqi counterpart Barham Saleh agreed Wednesday in Davos, Switzerland on the need for a continued US military role in the country, the White House said.

"That's really kind of a government-to-government discussion on when we get back to full restoration of that partnership. They certainly have an interest in it, as do we," Grynkewich said.