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A top U.S. diplomat strayed from a long-held wartime taboo Wednesday when he reported that 10,000 ISIS militants had been killed in Iraq and Syria, raising questions about how that number was compiled and whether it accurately reflected a U.S.-led coalition's success against the Islamic terror group.

"We have seen a lot of losses within Daesh since the start of this campaign, more than 10,000," Deputy Secretary of State Antony Blinken said on France Inter radio, using an alternate name for ISIS. "It will end up having an impact."

Blinken made the remarks as a stand-in for Secretary of State John Kerry at a Paris meeting of coalition members, who are under increasing pressure to step up the nine-month-old airstrike campaign. In recent weeks, ISIS has taken control of several key locations, including the Iraq city of Ramadi.

On its face, the body count seemed an attempt to show that the coalition's work — nearly 4,000 airstrikes and counting — was having an impact.

"We are spending millions a day, so at some point you want to say what you've accomplished," said Patrick Skinner, director of special projects for the Soufan Group, a security consulting company.

Smoke rises after an airstrike by the U.S.-led coalition on Islamic State group positions in an eastern neighborhood of Ramadi, Iraq. AP

But body counts are considered a poor method of showing progress. Their frequent and futile use in the Vietnam War was among the conflict's many lessons.

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One reason such numbers aren't typically released publicly is that they are hard to prove.

The U.S. military, which officially does not not consider body counts as measures of success, hasn't explained how exactly it compiles ISIS casualties. Analysts believe that the numbers come from various sources, including satellite imagery, reports from partners on the ground, and claims from ISIS itself.

The 10,000 cited by Blinken was a classified estimate that the Department of Defense and the military did not intend to release. It was not clear why he decided to announce it on Wednesday.

Defense officials told NBC News that the "estimated of the number killed is correct but was not intended for release."

NBC News has been told the number is accurate only in the context of the much broader operations carried out by other ISIS opponents. That includes the Kurds, Shiite militias being armed and advised by Iran, Iraqi forces and Syrian forces.

Laith Alkhouri, of security consulting firm and NBC News partner Flashpoint Intelligence, said he didn't believe Blinken's number. The U.S. government hasn't shared any underlying evidence, such as incremental reports of ISIS deaths, to back it up, he said.

Flashpoint's own monitoring of jihadist reports doesn't reflect such a body count, Alkhouri said, also noting that the number doesn't reflect ISIS' recent string of military victories.

"The reality on the ground is that ISIS is capturing territory, not losing ground," he said.

Josh Earnest, a spokesman for President Barack Obama, said Wednesday that he had "no reason to believe" Blinken's number was inaccurate.

Even without boots on the ground, Earnest said, there are several ways the administration can get updates on casualties, including routine battle damage assessments, Iraqi military intelligence and the use of "intelligence surveillance reconnaissance equipment."

ISIS is known to replenish many of the fighters it loses in airstrikes. militant website via AP file

Earnest also told reporters that ISIS is "working really hard to replenish their ranks" by recruiting members from around the world.

But the total number of ISIS fighters is equally difficult to assess.

Intelligence officials figure it is anywhere between 20,000 and 30,000. That wide range is the best indication that any numbers are a broad estimate, which is why the military prefers not to quote them.

No matter the number, statistics do little to mask the more important issues: the conditions that allowed ISIS to develop, and its ability to attract recruits and supporters from around the world, including the U.S., Skinner said.

So while the body count may seem like good news, "materially it won't impact anything," he said.