As the campaign against the Islamic State begins to draw to a close, the terror group is down to an estimated 6,500 fighters, with only about 100 left in Raqqa, its former self-proclaimed capital in Syria, the U.S. military declared Tuesday.

"ISIS in Iraq and Syria are all but isolated in their quickly shrinking territory," said Col. Ryan Dillon, a U.S. military spokesman in Baghdad.

The assessment came on the day Raqqa was declared liberated by the commander of the U.S.-backed Kurdish-led militia that has been slowly clearing the city of Islamic State fighters over the past four months.

The U.S. military stopped short of declaring the battle for Raqqa over, but said the Islamic State was on the "verge of a devastating defeat" at the hands of the Syrian Democratic Forces, a mix of Kurdish and Arab fighters.

"We are aware of the reports that ISIS has been defeated in Raqqa," Dillon told reporters at the Pentagon. "However, clearance operations continue, and we expect our Syrian Democratic Force partners to hit pockets of resistance as the final parts of the city is cleared."

Dillon said about 350 Islamic State members had surrendered over the past few days, bringing the total number who surrendered to roughly 400, a sign he said that it was a broken force.

"Today in Syria, ISIS is losing its grip," Dillon said, while stressing the group is not yet defeated.

The focus now shifts to an area in Syria the U.S. military has dubbed "the MERV," short for the Middle Euphrates River Valley.

"Once Raqqa is complete, it will be up to the commander whether or not he reallocates forces to continue to push down into the Middle Euphrates River Valley where there still is ISIS-held territory," Dillon said.

Dillon also downplayed reports of an Iraqi government assault on the Kurdish-held town of Kirkuk, insisting that there has been a "peaceful handover" of the military airport. He also denied reports that Iranian militia, known as Popular Mobilization Units, have taken over the city.

"As far as the Popular Mobilization Units in the area, you know, we have not seen in and around Kirkuk any of these units going in to re-establish security and/or inside of Kirkuk itself," Dillon said.

The tensions between the Kurdish factions in the north and the central government in Baghdad over the future of the oil-rich region and the desire by most Kurds for an independent state has threatened the unity of efforts against the Islamic State, Pentagon officials have said.

"We continue to monitor the situation and our leaders, talking with counterparts in both the Iraqi security forces and the Peshmerga, urged all sides to avoid escalation," Dillon said. "These tensions distract from our unified fight against ISIS, which remains a very real threat here in Iraq."

And Dillon says even when the Islamic State is defeated militarily, the war will not be over.

"We know that there still is going to be the ideology and the continued insurgent activity as they devolve into that," Dillon said.