The US-led coalition fighting against the Islamic State (IS) has killed more than 6,000 of its militants, US diplomatic officials have revealed.

US ambassador to Iraq, Stuart Jones, told Al Arabiya television the air strike campaign against the group was having a "devastating impact" on its fighters and its territorial gains.

The figure has been calculated by US Central Command (Centcom), which is keeping a tally of fighters killed, CNN reported.

US Defence Secretary Chuck Hagel would not confirm the figures but admitted that "thousands" had been killed in the campaign.

"It's a measure but I don't think it's the measure," said Hagel when asked if the new estimate demonstrated progress in the battle to contain the threat from the group.

"I was in a war where we did body counts and we lost that one," he added, referring to his time serving in Vietnam with the US army.

US intelligence officials believe the group has between 9,000 and 18,000 fighters in its ranks in their self-proclaimed caliphate straddling the Syrian-Iraqi border.

The revelation comes a day after officials from member countries of the US-led coalition gathered in London for talks on how to counter the threat posed by the terror group.

Speaking to reporters at the conference, US Secretary of State John Kerry said that "more than 700 sq km" had been recaptured from IS in Iraq.

Talks focused on how the coalition can provide more military assistance to those fighting the group on the ground in Iraq and Syria, as well as humanitarian aid and thwarting the group's cash flows.