IS 'has lost 50,000 fighters' over two years

The senior official described the figure as a "conservative estimate".

The figure showed air power and a small number of US figures supporting local forces were having an impact, the official said.

The US has, however, repeatedly warned that IS can replace fighters rapidly.

The official on Thursday said that coalition air strikes could be intensified in places like Mosul, which Iraqi troops are now battling to recapture, but that had to be offset against the risk of civilian casualties.

The campaign was beginning to damage IS, the official said.

"I am not into morbid counts but that kind of volume matters, that kind of impact on the enemy."

The US has often been reluctant to provide figures on enemy casualties.

But in August, Lt Gen Sean MacFarland was quoted by the AP news agency as saying that about 45,000 enemy combatants had been killed.

In February, White House Press Secretary Josh Earnest said IS had about 25,000 fighters operating in Syria and Iraq, citing a US intelligence estimate.

IS has lost a lot of ground since it reached the high tide of its expansion in 2014, and is now under fire from Russian, Turkish, Iraqi, Syrian and Kurdish forces, as well as US and British air power.

It is now entrenched in Mosul and Raqqa and the Sunni Arab tribal heartland of the Euphrates river valley, which stretches from eastern Syria to western Iraq.

(BBC)