At least 50,000 Islamic State jihadists have been killed by the US-led coalition since it began operations in Iraq and Syria in late 2014, a senior US military official said Thursday.

A relentless operation using planes and drones from a dozen or so members of the anti-IS coalition since August 2014 has conducted some 16,000 air strikes against the jihadists in Iraq and Syria -- two-thirds of them in Iraq.

In addition, the coalition has provided training and weapons to local forces fighting IS.

"I am not into morbid counts but that kind of volume matters, that kind of impact on the enemy," the official said, calling the 50,000 number a conservative estimate.

The official, speaking on condition of anonymity, said the air campaign had been the "most pristine" ever in terms of avoiding civilian casualties, with almost all the bombs dropped so far being smart weapons that can be steered to a precise target.

The coalition tally of civilians killed in the operations is 173 -- though critics say the real figure is far higher.

The official said the coalition had diminished IS's ranks to such a level that the simultaneous attacks being waged on Mosul in Iraq and Raqa in Syria -- the jihadists last remaining major power centers -- have been possible.

Coalition spokesman Colonel John Dorrian said earlier that in Mosul, IS was turning to adolescent fighters as its hardcore warriors got wiped out.

"As this effort goes on with each passing day, Daesh has fewer fighters and fewer resources at their disposal," Dorrian said in a videocall, using an Arabic IS acronym.

He added the jihadists appeared to have run out of armored suicide car bombs, and estimated "many hundreds" of fighters had been killed in Mosul.

"It doesn't mean that it's not still an extraordinarily dangerous situation. They are not going to go quietly, but they are going to go."

The coalition has previously said it "does not use a casualty count as a measure of effectiveness in the campaign to ultimately defeat (IS) in Iraq and Syria".

Despite this assertion, such figures are periodically announced.

Airwars, a London-based collective of journalists and researchers, uses local sources, photographs and media accounts to keep a detailed list of every known coalition air strike.

They have praised Pentagon efforts at accountability compared with other actors in Syria such as Russia and the regime of President Bashar al-Assad. But the group says the number of likely civilian deaths from coalition strikes is 1,957 at a bare minimum.