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

President Barack Obama demanded that the world take action against the "cancer" of militant extremism after the execution 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 Wednesday that special forces had been sent to Syria over the summer to try to rescue people held hostage by the IS militants, reportedly including Foley.

"This operation involved air and ground components and was focused on a particular captor network within ISIL (IS)," Pentagon spokesman Rear Admiral John Kirby said in a statement, without confirming if Foley was among the captives. "Unfortunately, the mission was not successful because the hostages were not present at the targeted location." The White House said Obama had "authorized 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."

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 IS.

The man, speaking with a British accident, then paraded a second US reporter, Steven Sotloff, before the camera and said he, too, would die unless Obama changed course. In the five-minute video, Foley is seen kneeling on the ground, dressed in an orange outfit that resembles those worn by prisoners held at the US naval base at Guantanamo Bay.

Foley was kidnapped in northern Syria in November 2012 and his grisly murder has provoked revulsion and condemnation across the globe.

"When people harm Americans anywhere, we do what's necessary to see that justice is done," Obama said Wednesday as US jets continued to strike IS 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.

"We will be vigilant and we will be relentless... From governments and peoples across the Middle East, there has to be a common effort to extract this cancer so it does not spread," Obama said. British Prime Minister David Cameron broke off his holiday to convene urgent meetings of the threat posed by IS, with rising concerns about how many militants are walking Britain's streets.

"We have not identified the individual responsible, but from what we have seen, it looks increasingly likely that it is a British citizen," Cameron told reporters. "This is deeply shocking." Richard Barrett, former head of counterterrorism at foreign intelligence service MI6, said he believed the suspected killer would be brought to justice "sooner or later".

Interpol has called for a global response to the militant threat, with monitors covering the conflict in Syria saying the Islamic State has more than 50,000 fighters in that country alone, including about 20,000 foreigners. Interpol chief Ronald Noble said there should be aa "multilateral response against the terror threat posed by radicalized transnational fighters traveling to conflict zones in the Middle East."

The European Union joined international condemnation of Foley's "outrageous" beheading.

"Such forms of terrorism constitute one of the most serious threats to international peace and security and the EU is more committed than ever to support international efforts to fight terrorism," said a spokesman for foreign policy chief Catherine Ashton. French President Francois Hollande has called for an international conference on tackling the Islamic State.

Meanwhile, American warplanes have bombed militants in northern Iraq near the Mosul dam, the US military's Central Command said on Thursday.

"US military forces continued to attack ISIL (Islamic State) terrorists in support of Iraqi Security Force operations, using fighter and attack aircraft to conduct six airstrikes in the vicinity of the Mosul Dam," it said in a statement. The air raids were carried out over the last 24 hours, a US defense official, who spoke on condition of anonymity, told AFP.