The US says its special forces tried this northern summer to rescue American hostages held by Islamist militants in Syria.
News reports quoting government officials said on Wednesday the captives included James Foley, the American journalist whose graphic execution by an Islamic State of Iraq and the Levant (ISIL) jihadist was revealed in a video on Tuesday.
Foley was kidnapped in northern Syria in November 2012 and his grisly murder has provoked revulsion and condemnation.
The Pentagon confirmed the failed hostage-rescue operation but did not specify whether Foley was among the captives.
"This operation involved air and ground components and was focused on a particular captor network within ISIL," Pentagon spokesman Rear Admiral John Kirby said in a statement.
"Unfortunately, the mission was not successful because the hostages were not present at the targeted location."
The White House said in a statement that President Barack Obama "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 custody".
In the execution video, a black-clad militant said Foley, a 40-year-old freelance journalist, was killed to avenge US air strikes against his movement.
The man then paraded a second US reporter, Steven Sotloff, before the camera and said he would also die unless Obama changes course.
Obama demanded on Wednesday the world take action against the "cancer" of jihadist extremism in Iraq. ISIL, also known as ISIS and the Islamic State, has seized much of eastern Syria and northern Iraq.
As US jets continued to strike IS targets in Iraq, despite the threat hanging over Sotloff, Obama said: "When people harm Americans anywhere, we do what's necessary to see that justice is done."
Shortly after he spoke, the State Department asked for 300 more US troops to be sent to Iraq to protect US facilities.
Obama paid tribute to Foley and said the Islamic State must be defeated.
"Jim Foley's life stands in stark contrast to his killers," he said, branding the militants genocidal murderers who target civilians and subject women and children to "torture and rape and slavery.
"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."
US intelligence believes the video is genuine, and the British government held a crisis meeting to launch an investigation because Foley's executioner spoke English with a London accent.
"We have not identified the individual responsible, but from what we have seen, it looks increasingly likely that it is a British citizen," Prime Minister David Cameron told reporters. "This is deeply shocking."
Foley's parents, John and Diane, appeared on the lawn of their home in New Hampshire to pay tribute to their son - the oldest of five adult children - and call 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.
French President Francois Hollande said Thursday he was "outraged" at the execution of Foley by Islamic State jihadists, accusing them of barbarism.
In a significant shift from its usual policy, Germany said it was ready to send weapons to support Iraqi Kurds against IS.
In June, the group then known as the Islamic State in Iraq and the Levant declared the dawn of a Muslim caliphate and seized control of a swath 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 Erbil or to civilian refugees from Iraqi religious minority groups.
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