The Islamic State (ISIS) has reportedly been dealt a devastating blow with more than 25,000 of its fighters killed by coalition airstrikes over the past 20 months.

The air campaign has halved the number of jihadis in ISIS-controlled Syria and Iraq and dozens of the terror group's leaders have been eliminated, the Daily Express reports.

Among the casualties, the report says, was British executioner Mohammed Emwazi, aka Jihadi John, who was killed during a drone strike.

"Much like a boxer, Daesh (ISIS) has taken several hard blows to the midsection," Col. Steve Warren, the U.S. military spokesman for the U.S.-led global coalition against ISIS, described the status of the terror group in a recent interview.

"We believe the knees are getting weak, and the head is starting to drop, and they are beginning to feel the effect of the exceptional pressure we've placed on them over 20 months."

According to Warren, 600 ISIS fighters had been killed in the past three weeks alone, and recent precision drone strikes and covert Special Forces missions to take out senior leaders had left the extremists "paranoid and in chaos."

Also, the spokesperson said strikes on ISIS-held oilfields had slashed the terrorist group's cash flow by a third – leading to a 50 percent pay cut for its fighters.

Warren also described Emwazi's death last November in Raqqa as "a greasy spot on the ground."

"I watched the video. We found him when he was alone on the street, talking on the phone," he said.

Emwazi became the world's most wanted man after beheading British hostages Alan Henning, 47, and David Haines, 44, reports say.

Warren spoke after reports came out that the coalition mission to ''degrade and destroy ISIS" was "faltering," as almost two years of airstrikes had allegedly failed to destroy the army of fanatics.

Warren said this has been "the most precise" air campaign in the history of warfare.

He said the group had been forced to flee from 40 percent of the territory it once held in northern Iraq.

He said British jets are playing a crucial role in pounding key targets, with RAF Typhoon and Tornado jets, along with drones, destroying key bases operated by the terror group.

Latest reports say the U.S.-led coalition is now preparing to mount a final assault on the ISIS stronghold of Raqqa in Syria.