The United States has warned that the killers of American journalist James Foley will be brought to justice, as they continued to carry out air strikes against the Islamic State (IS) in Iraq on Thursday.

US Attorney General Eric Holder, stated that the Department of Justice had opened a “criminal investigation” into the killing, adding,

“Those who would perpetrate such acts need to understand something. This Justice Department, this Department of Defence, this nation – we have long memories and our reach is very wide. We will not forget what happened, and people will be held accountable, one way or the other.”

The American journalist, who was kidnapped in 2012, was being held captive by Islamic State militants, who released a video of his execution earlier this week in retaliation to US air strikes against the group.

The release of the footage has sparked an international manhunt for the militant who kills Foley in the video, with unconfirmed reports suggesting that he is from London or the South-East of England.

James Foley's employers, GlobalPost, earlier revealed that Islamic State had demanded a ransom of $132 million for his release. The US State Department though, stated that they “do not pay ransoms”, with spokesperson Marie Harf saying,

"We believe that paying ransoms or making concessions would both put all Americans overseas at greater risk for kidnapping and in harms way, but that ransoms would also fund and finance exactly the groups we are trying to degrade their capabilities."

Foley's execution video also shows a second journalist being held hostage, identified as Steven Sotloff, with IS warning of further executions if US strikes were to continue. Despite this, the US military confirmed strikes against IS targets continued on Thursday, bringing the total number of US air strikes in the current campaign to 90.

As the air strike campaign continued, US Defence Secretary Chuck Hagel stated on Thursday the group was “beyond anything we have ever seen” and “clearly pose a long-term threat”.