The US is using its most expensive fighter aircraft in the bombing campaign against Islamic State (Isis) in Iraq and Syria, at a daily cost of more than $9m (£5.7m) a day.

The soaring bill has now reached $2.75bn since the campaign began on 8 August. Sources predict that if airstrikes continue for three more years, the overall cost to America will be around $10bn, according to the Times.

The first breakdown of US costs, released by the Pentagon, show that two-thirds of the total bill has gone to the Air Force.

More than 14,000 bombs have been dropped from aircraft, which cost thousands of dollars to operate.

A further $200m has also been spent on secret ground missions since August, while the US House of Representative approved a $579bn defence spending bill.

The cause for the spiralling costs is the amount of fuel consumed by the F-22 Raptor stealth fighter which costs $68,000 an hour to fly – at three times the cost of other fighters.

Critics question the tactics of airstrikes, saying they have had little impact on Islamic State.

However, a senior US commander at the heart of the campaign thought IS has altered its operations.

"Early in the conflict Isis paraded themselves in open convoys… now they hide among the civilian population and employ decoys in an attempt to be untargetable," Major John Easton, the US Air Force Central Command tactics officer said.

The White House also announced another 450 advisers for Iraq, bringing the total military personnel to 3,500.

But in a BBC report, US officials were keen to stress that there would be no "boots on the ground" and that their mission is to train local forces to do the fighting.

General Martin Dempsey, chairman of the Joint Chiefs of Staff, said the US' intervention in Iraq could extend even further, saying that airstrikes, which would assist troops in reaching nearer the front lines, remained a future option.

He also raised the issue of establishing a network of US training hubs in northern Iraq.