UK Defense Secretary Michael Fallon told reporters Thursday in London that more than 25,000 ISIS, or Daesh, fighters had been killed in the military campaign, a number that is half that being reported by US officials.
"More than 25,000 Daesh fighters have now been killed," Fallon said while appearing alongside his American counterpart, Ash Carter, following a meeting of defense ministers involved in the ISIS battle.
The British number is significantly smaller than the figures US officials have released in recent days.
As the Obama administration prepares to hand off the fight against ISIS to President-elect Donald Trump, efforts have been made to trumpet the current successes in that effort.
A senior military official told CNN last week that as many as 50,000 ISIS fighters had been killed, calling that figure a conservative estimate.
Brett McGurk, the White House's special envoy for the counter-ISIS fight, said this week that 17,000 coalition airstrikes have helped reduce the terror group's ranks to between 12,000 and 15,000 "battle ready" fighters.
And the commander of the international coalition, US Lt. Gen. Steve Townsend, told reporters Wednesday that 2,500 ISIS operatives had been killed or badly wounded since mid October.
But asked about the discrepancy between the US and UK figures, multiple American officials told CNN that the Pentagon does not officially tally body counts.
A representative from the UK Ministry of Defense told CNN that while Fallon's figure was an estimate, the British government stood by his number.
The dramatic difference between the US and UK figures risks damaging recent American efforts to bolster the legacy of President Barack Obama in his final days as commander in chief.
During stops in Afghanistan, Bahrain, Iraq and Italy over the last week, Carter repeatedly heralded the coalition's success against ISIS, citing the recent targeted killing of three ISIS leaders and the ejection of ISIS from Sirte, Libya, once the terror group's largest stronghold outside Syria or Iraq.
But Carter's predecessor, Chuck Hagel, said Thursday that the practice of counting the number of enemies killed wasn't a particularly useful one.
"My policy has always been, don't release that kind of thing," Hagel told CNN's Wolf Blitzer.
Hagel, a veteran of the Vietnam War where the American military's enemy body count statistics were disparaged for being overly optimistic, criticized releasing the figures.
"Body counts. I mean, come on, did we learn anything from Vietnam?" he asked, adding, "Body counts make no sense."
CORRECTION: This story has been updated to accurately reflect the US source for figures on ISIS fighter deaths.