Story highlights Brett McGurk, the US special envoy to the anti-ISIS coalition, gave an update Tuesday

President Barack Obama will hand the anti-ISIS mission to President-elect Donald Trump

(CNN) At least 75% of ISIS fighters have been killed during the campaign of US-led airstrikes, according to US officials.

The US anti-ISIS envoy said the campaign has winnowed ISIS' ranks to between 12,000 and 15,000 "battle ready" fighters, a top US official said on Tuesday.

The figures mean the US and its coalition partners have taken out vastly more ISIS fighters in Iraq and Syria than currently remain on the battlefield, two years since the bombing campaign began. Last week a US official said the coalition had killed 50,000 militants since 2014.

But it's also clear Obama will depart office without fully quashing the terror group, leaving a still-significant battle for Donald Trump when he becomes commander in chief in January.

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