Story highlights The Obama administration has been trumpeting the successes against ISIS

But the UK is putting the number of fighters killed at only around 25,000

Washington (CNN) The US and UK have very different estimates of how many ISIS fighters have been killed since the international coalition began striking the terror group in 2014, casting doubt on efforts to measure the success of the campaign.

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.

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