More than '50,000 IS fighters killed' in US-led airstrikes

At least 50,000 Islamic State group militants have been killed by the US-led coalition since it began air strikes in Iraq and Syria in 2014, a Washington official has claimed.

The air war on IS since it overran large swathes of the two countries in June 2014 has seen some 16,000 air strikes by members of the international anti-IS coalition.

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

The official - speaking on condition of anonymity - claimed 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' ranks to such a level that the simultaneous attacks being waged on Mosul in Iraq and Raqqa in Syria - the hardine group's last remaining major power centres - 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 told AFP using an Arabic IS acronym.

He added the fighters appeared to have run out of armoured 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 oust IS. However, 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, but the group says the number of likely civilian deaths from coalition strikes is 1,957 at a bare minimum.

The coalition has also provided training and weapons to local forces fighting IS.