US special forces were sent into Syria this year to try to rescue American hostages held by Islamist 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 jihadist 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.
- 'Justice must be done' -
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 jihadists 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."
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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 Islamist 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 a "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.
Last week, EU foreign ministers held a rare summer meeting to coordinate the bloc's response to the jihadist onslaught and gave unanimous approval to the arming of Iraqi Kurd forces by individual member states.
French President Francois Hollande has called for an international conference on tackling the Islamic State.
"If the world doesn't organize regarding this group there will be other equally appalling images, which won't only concern journalists, they've crucified people," he said.
The president of the world's most populous Muslim-majority country, Indonesia, Susilo Bambang Yudhoyono, described the beheading as a "new wake-up call" to the world, saying IS actions were not only "embarrassing" to Islam but "humiliating".
- 'Jim wouldn't want us to hate' -
Foley's parents, John and Diane, paid tribute to their son and called for other hostages to be released.
"Jim would never want us to hate or be bitter. We cannot do that and we are just so very proud of Jimmy and we are praying for the strength to love like he did," Diane said.
The scale of the threat from the Islamic State became clear in June when the group, then known as the Islamic State in Iraq and the Levant, declared the dawn of a Muslim caliphate and seized control of large parts of eastern Syria and northern Iraq.
Obama reacted this month by ordering US warplanes to counter threats to US personnel in the Kurdish regional capital Arbil and to civilian refugees from Iraqi religious minority groups.
He has insisted the scope of the strikes would remain limited, but Iraqi officials and observers have argued that only foreign intervention can turn the tide on jihadist expansion in Iraq.
Shiite militias, federal soldiers, Kurdish troops and Sunni Arab tribes have been battling IS for weeks in some areas but have been unable to clinch a decisive victory.