At least three quarter of ISIS fighters have been killed during the campaign of airstrikes against the terror group, US officials have said.

The US anti-ISIS envoy said ISIS' ranks had fallen from more than 60,000 to between 12,000 and 15,000 'battle ready' fighters.

The bombing campaign against ISIS, which began two years ago, has taken out more fighters in Iraq and Syria than currently remain on the battlefield.

Last week a US official said the American-led coalition had killed 50,000 ISIS militants since 2014.

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A picture taken from the Turkish side of the border shows smoke rising after attacks by war planes during an operation against Islamic State in Syria

Speaking at the White House Tuesday, Brett McGurk, the US special envoy to the anti-ISIS coalition, predicted the number of ISIS fighters would continue to decline, as the terror group is no longer able to replenish its members.

'The number of battle-ready fighters inside Iraq and Syria is now at its lowest point that it's ever been,' McGurk said, according to CNN.

The aerial bombing campaign has conducted 17,000 strikes against ISIS targets, with the most recent including three ISIS leaders the US says were responsible for plotting the attacks in Paris and Brussels last year and earlier this year.

The targets were taken out in Raqqa, ISIS' self-declared capital in Syria.

Smoke rises over Bartila in the east of Mosul, Iraq during clashes with Islamic State militants

Attempts to take out ISIS leader Abu Bakr al-Baghdadi, who appeared on a recent audiotape, have as yet been unsuccessful

However, he said attempts to take out ISIS leader Abu Bakr al-Baghdadi, who appeared on a recent audiotape, had not been successful.

'Issuing audiotapes deep in hiding is not really a sign of a confident leader, particularly in today's media age,' McGurk said.

'So eventually, we will find and eliminate him as well, but the leadership ranks continue to diminish.'