Defense Secretary Chuck Hagel Charles (Chuck) Timothy HagelWhile our foes deploy hypersonic weapons, Washington debates about funding Hillicon Valley: Democrats request counterintelligence briefing | New pressure for election funding | Republicans urge retaliation against Chinese hackers National security leaders, advocacy groups urge Congress to send election funds to states MORE is downplaying claims that 6,000 fighters from the Islamic State in Iraq and Syria (ISIS) have been killed in U.S.-led airstrikes.

Hagel said he had not seen "any verification" for that figure at a Pentagon briefing Thursday.

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"We do know that thousands of ISIL fighters have been killed, and we do know that some of ISIL's leadership have been killed," he added, using an alternate name for the group.

Hagel said that, regardless, a body count was not a reliable measure of the success of the international campaign against the terror group.

"Is that the measurement or a significant measurement of progress? It is a measurement. But I don't think it is 'the' measurement," he said.

"I mean, I — I was in a war where there was a lot of body counts every day. And we lost that war," Hagel continued, referring to the Vietnam War, when commanders touted the number of enemy killed as a measurement of success.

His comments came after U.S. Ambassador to Iraq Stuart Jones said Thursday that coalition airstrikes against ISIS had killed more than 6,000 of the group's fighters.

Jones also told Al Arabiya news network that airstrikes have "taken more than half" of the group's leadership.

The U.S. has been striking ISIS targets in Iraq since August and in Syria since September. Jones's comments were the first time a U.S. official publicly give an estimate on ISIS fighters killed.

Hagel said more important metrics are whether ISIS is on the defensive, having difficulty recruiting or accessing supplies, and able to communicate on the battlefield.

The Pentagon chief also said he did not agree with comments by Iraqi Prime Minister Haider Abadi on Wednesday that the U.S. was taking too long in providing weapons and training to Iraqi soldiers.

Last year, Hagel said, the Pentagon accelerated the shipment of more than 1,500 Hellfire missiles, more than 250 armored vehicles, and tens of thousands of small arms and ammunition.

In addition, he said three of four training sites for Iraqi troops have been set up, and the fourth one would be ready soon.

"We have a coalition of over 60 countries that have come together to help Iraq. And I think the prime minister might want to be a little more mindful of that," he said. "We are doing everything we can possibly do to help the Iraqis."

Hagel also hit back, saying the Abadi government is not moving as fast as the U.S. would like on political reforms to include Iraq's different ethnic and religious groups.

"Is he moving as fast as we would like? Probably not. But in governing, especially in democracies, and when there are elections, you have certain parameters," he said.

"But we've made it very clear, and I said this to him in my meeting, that is the one defining metric of success, ultimate success for his country because this is about the Iraqis. This isn't about the United States," Hagel added.