AKIPRESS.COM - It is reported an American journalist who was beheaded by Islamic State militants 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, News Talk said.

News of the unsuccessful attempt came as attempts are made to identify the man believed to have killed the American journalist. It has also been revealed that the journalist's family were told he would be killed a week before his beheading by an Islamic State (IS) militant.

The 40-year-old 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.

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)".