At least 50,000 Islamic State (Isis) fighters have been killed since America's "war on terror", a US military official has confirmed.

There has been a longstanding reluctance to disclose specific casualty rates among the terror group since the US-led coalition began the campaign to defeat IS (Daesh) in Iraq and Syria in 2014.

However, a senior Pentagon official confirmed the death toll to reporters on the condition of anonymity, due to the "sensitiveness of the information", CNN reported.

The official said the figure was a "conservative estimate".

It was also revealed that US special operations forces came "tantalisingly close" to capturing IS leader Abu Bakr al-Baghdadi in Raqqa.

Al Baghdadi's presence in the northern Syrian city was revealed to intelligence officials after IS operative and oil executive Abu Sayyaf was killed in a raid in Deir Ez-Zour in 2015 while his wife was captured, after she tried to escape disguised as a Yazidi sex slave.

After two days of questioning, she revealed she and her husband had been with al-Baghdadi in Raqqa.

The official said, "We had been tracking them in Raqqa, so we went back frantically to try to figure out where he [al-Baghdadi] was in Raqqa," but the trail was said to have gone cold.

US Secretary of Defence Ash Carter said on Saturday (10 December) that 200 extra troops will be deployed to Raqqa to take the IS stronghold.

"These uniquely skilled operators will join the 300 US special operations forces already in Syria, to continue organising, training, equipping, and otherwise enabling capable, motivated, local forces to take the fight to ISIL," Carter said, using an alternative acronym for IS.

"By combining our capabilities with those of our local partners, we've been squeezing ISIL by applying simultaneous pressure from all sides and across domains, through a series of deliberate actions to continue to build momentum," he said.

It comes as US-backed forces are pushing back the extremists in Mosul, Iraq and have liberated Sirte from IS control in Libya.