The U.S. military launched a secret mission this summer to rescue a number of Americans held captive in Syria by the Islamic State but failed to find them, senior White House officials said Wednesday night. The mission was carried out by several dozen special operations troops who were on the ground in Syria for a short time.

Announcement of the complicated mission came a day after the beheading of an American photojournalist James Wright Foley by the Islamic State (IS). He was one of the hostages the mission had hoped to extract.

The IS had demanded a multimillion-dollar ransom for Foley’s release, which the U.S. refused, the New York Times reported Thursday. The group has threatened to kill at least three other hostages it holds if its demands are not met, the report added.

Officials said the rescue mission was authorized after intelligence agencies believed they had identified where in Syria the hostages were being held. But the several dozen special operations forces dropped by aircraft into Syria did not find them at that location and engaged in a firefight with IS militants before departing.

The operation “involved air and ground components and was focused on a particular captor network within ISIL,” said Pentagon press secretary Rear Adm. John Kirby in a statement. (The Islamic State of Iraq and the Levant is a name previously used by the Islamic State.)

“The U.S. government had what we believed was sufficient intelligence, and when the opportunity presented itself, the president authorized the Department of Defense to move aggressively to recover our citizens,” said Lisa Monaco, Barack Obama’s top counterterrorism adviser, in a statement. “Unfortunately, that mission was ultimately not successful because the hostages were not present.”

The mission, authorized by Obama, took place in early July, the New York Times reported. Administration officials did not specify when or where the operation took place, citing the need to protect operational details to preserve the ability to carry out future missions. They said 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. There had been no plans to disclose the mission, but it was made public “when it was clear a number of media outlets were preparing to report on the operation and that we would have no choice but to acknowledge it,” according to a statement from the National Security Council.

The Obama administration detailed the mission one day after IS fighters posted on social media that they had beheaded Foley and would kill a second American journalist, Steven Joel Sotloff, who went missing in Syria in August 2013.

It is unclear how many Americans the special forces attempted to rescue. While the officials who described the mission would not provide an exact number, other U.S. officials, who were not authorized to speak publicly, have said Foley was one of at least four Americans held in Syria. One of them is freelance journalist Austin Tice, who disappeared in Syria in August 2012 and is believed to be in the custody of government forces in Syria.

The IS said the murder of Foley, who had been missing since being abducted in Syria in November 2012, was in retribution for U.S. airstrikes on its forces in northern Iraq.

The U.S. launched a new barrage of airstrikes against IS targets in Syria on Wednesday despite the group’s threats. The Obama administration did not rule out the prospect of a military operation in Syria to bring to justice those responsible for Foley’s death.

The disclosure of the rescue mission marks the first time the U.S. has revealed that American military personnel have been on the ground in Syria since civil war there broke out more than three years ago. Obama has resisted calls to insert the U.S. military in Syria’s war — a cautious approach that his critics say has allowed the IS to strengthen there and make gains across the border in Iraq.

The IS is an Al-Qaeda breakaway group that has established what it calls a caliphate across swaths of territory in Iraq and Syria. Obama last week ordered airstrikes in support of Kurdish forces fighting the IS to prevent it from overrunning Kurdish-controlled territory in northern Iraq. The movement's fighters have conducted mass executions of Iraqis of the Yazidi faith.

Al Jazeera and wire services