Secret mission to rescue Foley

US special forces were sent into Syria earlier this year to try to rescue American hostages held by Islamist militants, US officials said as international revulsion mounted over the beheading of journalist James Foley.

President Barack Obama demanded the world take action against the "cancer" of jihadist extremism after the murder of the American journalist by Islamic State militants who have seized swathes of Syria and Iraq.

Outraged US allies have pledged to help in the battle against the Islamic State, sending in weapons and other aid to Kurdish forces fighting the extremists in northern Iraq, while Washington pressed on with air strikes.

US government officials confirmed on Wednesday that special forces had been sent to Syria over the summer to try to rescue people held hostage by the Islamic State militants, reportedly including Foley. Pentagon spokesman Rear Admiral John Kirby said: "This operation involved air and ground components and was focused on a particular captor network within Islamic State."

Kirby did not confirm if Foley had been among the captives the mission hoped to rescue.

"Unfortunately, the mission was not successful because the hostages were not present at the targeted location."

According to the New York Times, the US Army's elite Delta Force took part in the foiled rescue operation.

The White House said Obama had "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 Islamic State custody."

In the execution video, a black-clad militant said that Foley, a 40-year-old freelance journalist, was killed to avenge US air strikes against the terrorist group.

The man, speaking with a British accent, then paraded a second US reporter, Steven Sotloff, before the camera and said he, too, would die unless Obama changed course.

Foley was kidnapped in northern Syria in November 2012.

"When people harm Americans anywhere, we do what's necessary to see that justice is done," Obama said on Wednesday as US jets continued to strike Islamic State targets in Iraq despite the threat hanging over Sotloff.

The State Department has asked for 300 more US troops to be sent to Iraq to protect US facilities. "There has to be a common effort to extract this cancer so it does not spread," Obama said.

Susilo Bambang Yudhoyono, the president of Indonesia, has called the actions of Islamic State militants "embarrassing" to the religion and urged Islamic leaders to unite in tackling extremism.