The Pentagon confirmed Friday its estimate that U.S.-led coalition airstrikes have killed about 6,000 ISIS fighters since the air campaign began in August.

Pentagon Press Secretary John Kirby insisted that the figure is an estimate and not the key “metric of success,” but said coalition airstrikes and coordination with Iraqi and Kurdish forces have been effective in pushing back ISIS and putting the militant group on the defensive.

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The number of ISIS fighters in Iraq and Syria continues to fluctuate, but the deaths account for about 20 to 30% of the total ISIS force, based on Pentagon estimates that ISIS is about 20,000 to 30,000 fighters strong.

“It’s not the metric of success and it’s not a metric that we’re going to hang our hat on when it comes to the success of the strategy,” Kirby said.

If the Pentagon estimates are in line with the a separate calculation of ISIS casualties only in Syria from the Syrian Observatory of Human Rights, the coalition effort is mostly degrading ISIS in Iraq. The human rights group documented more than 1,200 ISIS casualties in Syria since coalition airstrikes started in that country in September, the group announced Friday.

The U.S. effort in Iraq started a month earlier and the U.S. has closely coordinated with local forces on the ground in Iraq, unlike in Syria.

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But even in Iraq, the slow territorial gains by American-aided Iraqi soldiers add important context to the estimated number fighters killed.

With the help of the U.S.-led coalition, a force of mostly Kurdish Pesh Merga fighters have retaken about 700 square kilometers of ISIS-held territory in northern Iraq out of about 55,000 square kilometers of land under ISIS control. Iraqi forces control about the same area of territory and Kurdish forces control about 20,000 more square miles more, according to assessments of populated and significant land.

And while coalition forces have only taken a small bite of ISIS-controlled territory in Iraq, Kirby insisted that the U.S. is making progress and that the land gains are the result of just five months of coalition airstrikes and training in a multi-year military campaign.

Beyond territory gains and ISIS casualties as measures of success, Kirby pointed to changes in how ISIS militants are operating and communicating. The militant group is now operating in a more defensive posture, he said, leading fewer offensive charges and recruiting more children as fighters, a sign they could be struggling with their numbers.

ISIS is acting more to defend their territory gains and supply lines, Kirby said, calling the group “retrenched.”

“They’re hiding more. If they’re hiding more and they’re constrained then they’re not able to enact the same influence” Kirby said. “It’s really about how they’re behaving and how they’re changing you can glean a lot about an enemy.”

Kirby acknowledged that the U.S. is “mindful” that ISIS is still a “potent force” in both Iraq and Syria.

“This is going to continue to take some time,” he said.