US tried but failed to rescue aid worker: Obama

US President Barack Obama has said that the recently deceased aid worker Kayla Mueller was among the American hostages captured by the terrorist group, Islamic State (IS), who his government tried to rescue last year in an unsuccessful operation.

"We devoted enormous resources, always devote enormous resources to freeing captives or hostages anywhere in the world," Obama said Tuesday in an interview with the online Buzzfeed News.

"And I deployed an entire operation -- at significant risk -- to rescue not only her but the other individuals who had been held, and probably missed them by a day or two, precisely because we had that commitment," said Obama.

The US revealed in August that it had launched a secret mission in Syria to secure the release of several hostages, including the American journalist James Foley, abducted in Syria in 2012 and beheaded in 2014, but the mission failed because it did not find the correct location.

Obama said that among the hostages was Mueller, 26, whose death was confirmed Tuesday by her family and the White House.

The president defended the US policy of not paying ransom for the release of American hostages abroad, while Mueller's parents were raising funds for that purpose.

"We will do everything we can short of providing an incentive for future Americans to be caught," Obama said.

He also said that although other governments can decide to pay ransom to organisations like the IS, the US would remain firm in its policy in this regard.

"The reason is that once we start doing that, not only are we financing their slaughter of innocent people and strengthening their organisation, but we're actually making Americans even greater targets for future kidnappings," he added.

Obama confessed that he was "heartbroken" to learn of the death of Mueller who he described as "an outstanding young woman" whose "spirit will live on".

Mueller is the fourth American hostage killed by the IS and the White House stated Tuesday that it had evidence of at least another American retained in the Middle East, although it failed to specify the country or the identity of that person's captors.