A criminal investigation has been launched into the beheading of journalist James Foley, US Attorney-General Eric Holder said.

A video was released of the US reporter being killed by a militant from the Islamic State (IS) in which the group said it was acting in retaliation for US airstrikes targeting its fighters in Iraq.

Counter-terrorism police in the UK have already begun efforts to identify the man shown in the video beheading Mr Foley.

Although his face is covered, he speaks with a British accent and other people previously held hostage by the group have suggested he may be a jihadist known as "John" who was part of a group guarding captives in Syria.

It is also reported that Mr Foley was among a group of hostages the US military tried to free this summer. The Pentagon has revealed a secret mission took place recently to rescue American citizens held in Syria, but they were not located.

He is alleged to have been murdered in a propaganda video issued by the Islamist group, which warned another captured American, Steven Sotloff, would also be killed unless the US called off airstrikes in Iraq.

US President Barack Obama's Homeland Security adviser, Lisa Monaco, said in a statement that authorities had recently become increasingly concerned over the plight of hostages in Syria.

"The President authorised action at this time because it was the national security team's assessment that these hostages were in danger with each passing day in ISIL (IS) custody," she said.

"The US government had what we believed was sufficient intelligence, and when the opportunity presented itself, the President authorised the Department of Defense to move aggressively to recover our citizens. Unfortunately, that mission was ultimately not successful because the hostages were not present."

Pentagon spokesman Rear Admiral John Kirby said the operation "involved air and ground components and was focused on a particular captor network within ISIL (IS)".

Warning from kidnappers

Meanwhile, Mr Foley's employers said they received an e-mail last Wednesday hitting out at US airstrikes and saying he would be "executed".

Philip Balboni, President of the Global Post news website, said "On Wednesday night last week the Foley family received an email from the kidnappers that was full of rage against the United States bombing and they stated that they would execute Jim."

"Obviously, we hoped and prayed that that would not be the case. We communicated as quickly as we could, with the captors; pleaded with them for mercy."

Mr Balboni said the Global Post spent millions on efforts to bring Mr Foley home, including the hiring of an international security firm.

The militant in the propaganda video said Mr Foley (40) was being killed because of US airstrikes against fighters from Islamic State, formerly known as ISIS or ISIL.

Originally published 06:06