Air strikes have killed three senior IS leaders - including the group's financial minister -and an estimated 350 fighters holed up in the Iraqi city of Ramadi, a US military spokesman has said.

Colonel Steve Warren, spokesman for the US military command in Baghdad, announced the deaths of Abu Salah, known as IS' financial minister, Abu Maryam, an "enforcer and senior leader of their [ IS] extortion network”, and Abu Rahman al-Tunisi, an "executive officer" who handles the transfer of information, people and weapons.

Col Warren told reporters at the Pentagon: "Killing him and his predecessors exhausts the knowledge and talent needed to coordinate funding within the organisation."

He said all three were killed in late November during air strikes near the town ofTal Afar, in northern Iraq.

Acting undersecretary for terrorism and financial intelligence, Adam Szubin, said some of IS' funds came from looting between $500m and $lbn from bank vaults captured in Iraq and Syria, and through extorting the local population. According to the Financial Times, he told Chatham House in London the extremists were selling "a great deal of oil to the Assad regime" but the ramped-up bombing campaign on their oil infrastructure over the last month was "markedly degrading" their funds.

Col Warren also said recent airstrikes had killed an estimated 350 IS fighters in the city of Ramadi in western Iraq, which was captured by IS in May.

The announcement suggested the militants have now lost as much as half their defending force.

Col Warren said there had been an estimated 600 to 1,000 IS fighters inside Ramadi, but officials are reluctant to predict how long it may take to reclaim the city.

Defence Secretary Ash Carter admitted the retaking of Ramadi was "disappointingly slow".

"I am certain it will fall, and we will assist in the making of it fall," he said.

He added that the US would be willing to commit US army Apache attack helicopters to the battle for Ramadi, if the Iraqi government requests it and if it would "make a strategically decisive difference".

Asked about that comment later, Col Warren said, "Apache helicopters are ready," if Washington and Baghdad give the go-ahead.

He said the US is prepared to use Apache attack helicopters in Ramadi if requested by Washington and Baghdad, according to CBS News.