A US secret military mission had tried but failed to free US reporter James Foley and other American hostages in Syria, US officials have said.
Their comments come after a video of Foley's beheading by Islamic State (IS) militants appeared on Tuesday.
IS said Foley's death was revenge for US air strikes on its fighters in Iraq.
US President Barack Obama condemned the killing as "an act of violence that shocks the conscience of the entire world".
He compared IS militants, who control large parts of Syria and Iraq to a "cancer" and said the group's ideology was "bankrupt".
The UN, UK and others have also expressed abhorrence at the video.
Mr Foley's mother Diane said he "gave his life trying to expose the world to the suffering of the Syrian people".
The Committee to Protect Journalists said it was "extremely concerned for all journalists" still held by IS, describing Syria as "the world's most dangerous place to be a reporter".
In a statement on Wednesday, the Pentagon said the US had "attempted a rescue operation recently to free a number of American hostages held in Syria".
It said the operation "involved air and ground components and was focused on a particular captor network within ISIL" (the former name of IS).
"Unfortunately, the mission was not successful because the hostages were not present at the targeted location."
The statement did not specify whether the operation had intended to rescue Foley, who was kidnapped in Syria in November 2012.
However, senior Obama administration officials - speaking on the condition of anonymity - confirmed this.
They said that several dozen special troops had been dropped by aircraft into Syria in recent weeks to try to rescue US hostages, including Foley.
They added that the troops had been engaged in a firefight with IS militants, killing a number of them. No Americans were killed.
Foley, 40, had reported extensively across the Middle East, working for US publication GlobalPost and other media outlets including French news agency AFP.
Analysis: BBC Security Correspondent Frank Gardner
The race to identify James Foley's killer has been taking many forms.
On the databases of MI5, the UK security service, in the records of the Police Counter-Terrorism Unit and out in the digital forums and social media postings of the internet, clues are being examined and assessed.
The man who murdered the American journalist in cold blood is thought to have probably come from London or south-east England. He is presumed to have travelled to Syria some time during the last three years and he will be well known to some in jihadist circles.
Unconfirmed reports say he is one of a small handful of Britons tasked with guarding Islamic State's western captives. The militant who appears in the video is careful to keep much of his face hidden but the voice on the tape is distinctive and analysts believe it is only a matter of time before he is identified.
Bringing him to justice though, will be an altogether more difficult proposition.
'Warning to Obama'
In the IS video, titled A Message to America, a man identified as James Foley is dressed in an orange jumpsuit, kneeling in desert-like terrain beside an armed man dressed in black.
He gives a message to his family and links his imminent death to the US government's bombing campaign of IS targets in Iraq.
Clearly under duress, he says: "I call on my friends, family and loved ones to rise up against my real killers, the US government, for what will happen to me is only a result of their complacency and criminality."
Then the masked militant - who speaks with a British accent - delivers a warning to the US government: "Any attempt by you, Obama, to deny the Muslims their rights of living in safety under the Islamic caliphate will result in the bloodshed of your people."
After he speaks, the militant appears to start cutting at his captive's neck before the video fades to black. His body is then seen on the ground.
Another captive, identified as American journalist Steven Sotloff, is shown at the end, with the warning that his fate depends on President Obama's next move.
Mr Sotloff was abducted in northern Syria a year ago.
- Formed out of al-Qaeda in Iraq (AQI) in 2013, IS captured Raqqa in eastern Syria
- By early 2014 it controlled Falluja in western Iraq
- It has since captured broad swathes of Iraq, seizing Mosul in the north in June and the Mosul dam in August
- The violence has displaced an estimated 1.2 million people in Iraq alone
- Pursuing an extreme form of Sunni Islam, IS has persecuted non-Muslims such as Yazidis and Christians, as well as Shia Muslims, whom it regards as heretics
- In July alone, IS expanded dramatically, recruiting some 6,300 new fighters largely in Raqqa, an activist monitoring group said