"We very much see ISIL largely in a defensive posture inside Iraq, that whatever momentum they had been enjoying has been halted, has been blunted. That has stayed steady over the last couple of weeks," said Pentagon spokesman Rear Admiral John Kirby.

Briefing reporters on Tuesday, Kirby said the change is occurring mainly in Iraq for now.

"We know we have destroyed hundreds and hundreds of vehicles, artillery positions, checkpoints. We know that we have killed hundreds of their forces," he said, though he could not say specifically how many had been killed. "We don't have the ability to count every nose that we schwack."

For months, ISIS moved with lightning speed across northern and western Iraq, taking the town of Mosul, moving into Fallujah and Ramadi in al Anbar province. But for the last two to three weeks, ISIS has not taken significant new ground, the U.S. says. Instead ISIS has had to go on defense, trying to hold on to what it has.

"They are having trouble paying their fighters, they are having trouble governing, they don't have the leaders to actually govern the spaces they have attacked and taken over," says CNN military analyst retired Lt. General Mark Hertling.

But there are many caveats.

A U.S. intelligence official tells CNN that ISIS' "core capabilities remain fundamentally unchanged."

The official notes that while the group has suffered battlefield losses, it's mainly mid-level fighters and "it's likely most have been replaced."

Additionally, the group is still using a huge safe haven area in Syria "from which to train, plan and conduct attacks. The group can freely funnel men and material between Iraq and Syria."

And, the official added, "It's the wealthiest terrorist group in the history of terrorism. They've got hundreds of millions of dollars, plenty of recruits and a slick propaganda machine that continues to generate more of both."

That said, both the U.S. intelligence community and the military agree that the coalition has blunted ISIS' momentum.

"The rate at which the group was taking territory has decreased. The group has lost turf in Iraq, although its footprint in Syria remains the same," the official said.

Also, ISIS is losing popularity with some of the population under its control.

"The group is struggling to provide services in some areas, and some tribes have turned against [them]," the official said.

Also, possibly slowing down is the number of new foreign fighters traveling to Iraq and Syria, according to a senior U.S. official. The U.S. believes about 150 people from the U.S. have traveled, or attempted to travel, to the region to fight -- part of the 3,000 westerners from 90 countries.

But with ISIS still able to launch attacks at places like al Asad airbase where 320 Marines are located, an urgent effort is underway there and at other bases to train Iraqi units. The U.S. now plans to sell Iraq 175 M1A1 Abrams battle tanks and 1,000 HMVEEs for nearly $3 billion. It's sending 250 MRAP mine-resistant vehicles and 10,000 M-16 rifles. All of this aimed at getting Iraqi forces not just trained, but armed and willing to fight ISIS in order to take their country back.

For the first time, the Pentagon revealed it's investigating several allegations of potential civilian casualties caused by airstrikes in Syria and Iraq. Outside monitoring groups claim hundreds of civilians may have been killed in such strikes.

(CNN)