US mission to rescue Foley failed
US special operations troops went on a secret mission to Syria this summer to rescue American hostages, including journalist James Foley.
But the forces could not find the hostages, and engaged in a firefight with Islamic State militants before departing, the White House said.
President Barack Obama authorised the mission after intelligence agencies believed they had identified the location inside Syria where the hostages were being held.
Several dozen special operations forces were dropped by aircraft into Syria, according to Lisa Monaco, the president's senior counter-terrorism adviser.
"The US government had what we believed was sufficient intelligence, and when the opportunity presented itself, the president authorised the department of defence to move aggressively to recover our citizens," she said.
"Unfortunately, that mission was ultimately not successful because the hostages were not present."
Officials disclosed the rescue operation a day after the militants released a video showing the beheading of Mr Foley and threatened to kill a second hostage, Steven Sotloff, if US airstrikes against the militants in Iraq continued.
Despite the militants' threats, the US launched a new barrage of airstrikes against Islamic State targets in Syria yesterday.
The Obama administration did not rule out the prospect of a military operation in Syria to bring those responsible for Mr Foley's death to justice.
The disclosure of the rescue mission marks the first time the US has revealed that American military personnel have been on the ground in Syria since a bloody civil war there broke out more than three years ago.
Mr Obama has resisted calls to insert the US military in the middle of Syria's war, a cautious approach his critics say has allowed the Islamic State to strengthen there and make gains across the border in Iraq.
A number of militants, but no Americans, were killed in the firefight in Syria. One American suffered a minor injury when an aircraft was hit, officials said.
"As we have said repeatedly, the United States government is committed to the safety and well-being of its citizens, particularly those suffering in captivity," Pentagon spokesman Rear Admiral John Kirby said.
"In this case, we put the best of the United States military in harm's way to try and bring our citizens home.
"The United States government uses the full breadth of our military, intelligence and diplomatic capabilities to bring people home whenever we can.
"The United States will not tolerate the abduction of our people, and will work tirelessly to secure the safety of our citizens and to hold their captors accountable."
It is unclear how many Americans the special forces attempted to rescue in Syria. Officials have said Mr Foley was one of at least four Americans held in Syria.
Like Mr Foley, two others are believed to have been kidnapped by the Islamic State group. The fourth, freelance journalist Austin Tice, disappeared in Syria in August 2012 and is believed to be in the custody of government forces in Syria.
Administration officials would not say specifically when or where the operation took place, citing the need to protect operational details in order to preserve the ability to carry out future rescue missions.
They did say that nearly every branch of the military was involved and that the special forces on the ground were supported from the air by fixed wing, rotary and surveillance aircraft.
Mr Obama has authorised previous military missions to rescue hostages.
In 2009, Navy SEAL snipers carried out a daring sea operation to rescue an American ship captain held by Somali pirates in a lifeboat. And in 2012, special operations forces successfully rescued an American and Dutch aid worked held in Somalia.
Meanwhile t he chief executive officer of a Boston-based news organisation says Mr Foley's kidnappers last week threatened to kill him in response to US bombings in Iraq.
Mr Foley was freelancing for GlobalPost when he disappeared in Syria.
GlobalPost chief executive said that the threatening email sent to Mr Foley's family was "full of rage" but made no demands. He says the kidnappers ignored pleas for mercy.
Mr Balboni says the company spent millions of dollars on efforts to bring Mr Foley home including hiring an international security firm.
The 40-year-old journalist from New Hampshire was abducted in northern Syria in November 2012 and had not been heard from since.
FBI Director James Comey says the Islamic State extremists who executed Mr Foley are "savages" and that the agency is helping to investigate the killing.
Mr Comey says the FBI has been investigating American hostages in Syria for two years and that "these savages have turned that into a homicide investigation".
He said the FBI has not concluded when Mr Foley was killed.