Washington (CNN) The coalition fighting ISIS has killed more than 6,000 fighters, including half of the top command of the terror group, U.S. diplomatic officials said Thursday.

The number of fighters killed has not been publicly discussed before but was disclosed by the U.S. ambassador to Iraq Stuart Jones, who told Al Arabiya television earlier in the day that an estimated 6,000 fighters have been killed. Jones said the military effort was having a "devastating" impact on ISIS.

The estimate was calculated by U.S. Central Command and finds ISIS fighters have been killed in Iraq and Syria by coalition airstrikes, according to a U.S. military official. CENTCOM has kept a running estimate of fighters killed, but has not made it public.

U.S. intelligence estimates that ISIS has a total force of somewhere between 9,000 to 18,000 fighters. However, it is also believed the group can draw on thousands of other fighters whose loyalty shifts and could muster a force upwards of 31,000 total.

Defense Secretary Chuck Hagel, would not confirm the 6,000 deaths estimate, instead saying "thousands" have been killed.

On whether the body count is a sign of progress, Hagel said, "It's a measure but I don't think it's the measure."

"I was in a war where we did body counts and we lost that one," Hagel said, referring to his service in Vietnam.

Until now, the Pentagon has stayed away discussing the matter, other than to estimate that thousands of fighters may have been killed. Speaking to reporters Thursday, Rear Admiral John Kirby was adamant that the US is not keeping a "body count," and said it would be wrong to state that there is such a count. He called it a 'tally" and said the notion of a body count suggests Vietnam War era statistics. In that war the Pentagon offered body counts as a measure of its success against the Viet Cong. Kirby said the tally was not aimed at showing any metric of success against ISIS.

All of this comes after Iraq has criticized the U.S. for not doing enough to help their fight against ISIS. The U.S. has long said airstrikes are aimed at degrading ISIS as a threat, but would not by themselves get rid of the organization.

Secretary of State Kerry, speaking to reporters in London, echoed the U.S. ambassador in saying the strikes have "halted" ISIS momentum, and reclaimed "more than 700 square kilometers" from ISIS in Iraq.

In Iraq, airstrikes around Mosul have been stepped up significantly in support of Peshmerga fighters on the advance in the region. The effort now is to cut a key ISIS supply line into Mosul, a US military official said.

The official stressed the US cannot confirm the exact number but has based its calculation based on pilot reports, and other intelligence gathered about a target before and after a strike.