WASHINGTON — President Barack Obama on Wednesday vowed justice for the Islamist killers of American journalist James Foley, as officials revealed that U.S. forces had launched a secret raid inside Syria last month to rescue him and other captives only to find they had been moved.

As Obama spoke, U.S. forces launched 14 new airstrikes against the Islamic State, in defiance of the group’s threat Tuesday to kill a second American journalist if Obama didn’t end the attacks. European leaders, meanwhile, moved toward a more aggressive stand.

Obama stressed that U.S. airstrikes would continue, and he indicated that the United States would pursue Foley’s killers.

“When people harm Americans, anywhere, we do what’s necessary to see that justice is done. And we act against ISIL, standing alongside others,” he said, using the official U.S. acronym for the Islamic State, a spinoff of al-Qaida.

White House officials pointed to last month’s attempt to underscore that the U.S. would spare no effort.

A team of several dozen U.S. special forces operators entered Syria over the July 4 weekend, only to discover once on the ground that the captives had been moved, senior U.S. officials said Wednesday.

The raid is the first known instance of U.S. troops crossing into Syria since the civil war erupted there in mid-2012. The disclosure offered insight into how much intelligence the United States had gathered on the whereabouts of Foley and the undisclosed number of other Americans held by the Islamic State.

Obama authorized the raid after several threads of intelligence pinpointed the location of the prisoners, senior administration officials said.

“The president authorized action at this time because it was the national security team’s assessment that these hostages were in danger with each passing day,” said Lisa Monaco, an assistant national security adviser.

The force arrived at the site by air, conducted a search on foot and left after discovering that the hostages were not there, the officials said. The U.S. troops came under fire as they departed and shot back, and one special forces operator was injured aboard one of the departing aircraft.

“We do believe that there were a good number of ISIL casualties,” a senior administration official said.

The officials declined to disclose how long the operators were on the ground or where the raid took place.

Many experts believe that Foley and other foreigners were being held in northern Syria, large parts of which were overrun in 2013 by the Islamic State, which established its headquarters in Raqqa, the provincial capital of a northeastern Syrian province of the same name.

An Islamic State aide told McClatchy on July 4 that helicopter-borne American forces flew into the al Ikairsha area of Raqqa province and stormed an Islamic State base named Camp Osama bin Laden.

The aide, who spoke on condition of anonymity, said that the U.S. raiding party killed five militants and injured many more before departing.

The Islamic State viewed the attack as “strange” and saw it as the start of a “sacred war” between the group and “the grandchildren of apes and pigs,” the aide said.

While Obama has vowed U.S. combat troops won’t return to Iraq, the Pentagon is considering a request from the State Department to send additional U.S. military to Iraq to boost security for U.S. personnel based there, said a U.S. official who requested anonymity because he wasn’t authorized to discuss the issue publicly.

The request was up to 300 additional security personnel in and around Baghdad, the official said, adding “there are no indications it was in response to a specific threat against Americans in Baghdad.”

Defense Secretary Chuck Hagel would have to approve the request, but he has not personally received it yet, said the U.S. official. Moreover, he continued, “if past is prologue,” the administration would seek congressional authorization for the additional troops.

Some 749 U.S. troops have been sent to Iraq since the Islamic State overran the northern city of Mosul on June 10 and then advanced south to the outskirts of Baghdad, overrunning roughly one-half of Iraq in cooperation with Sunni Muslim tribes alienated by the sectarian policies of the majority Shiite-dominated government.

Most of those U.S. personnel were posted to a joint operations center in the Kurdish regional capital of Irbil that’s coordinating U.S. airstrikes and Iraqi ground operations against the Islamic State.

There was a sense within part of the Pentagon that the decision to execute Foley and threaten to kill a second American journalist, Steven Joel Sotloff, suggested that the U.S. airstrikes were having an impact and that the video represented an attempt to stop them.

Yet other Pentagon officials feared that Foley’s death was an effort by the Islamic State to further draw the United States into the Iraq crisis as a way of bolstering its claim that the group is defending from outside attacks.

Obama said that he’d spoken to Foley’s family in New Hampshire, telling them, “We are all heartbroken at their loss.”

Foley’s life, Obama said, “stands in stark contrast to his killers,” whom he said have “rampaged across cities and villages, killing innocent, unarmed civilians in cowardly acts of violence.”

Secretary of State John Kerry issued a harsher condemnation. “ISIL and the wickedness it represents must be destroyed, and those responsible for this heinous, vicious atrocity will be held accountable,” Kerry vowed.

Foley’s grieving parents spoke with reporters for more than an hour outside their home, telling reporters their son had wanted to “bear witness to all the suffering.”

“He had an incredible heart and he always cared about people who were suffering — and that’s why he went back,” said his mother, Diane Foley.

Foley’s father, John, said he asked Obama to “do whatever he could possibly do” to save the lives of Sotloff and others being held.

“It haunts me, how much pain he was in and how cruel this method of execution is,” his father said. “He was courageous to the end. … We believe he was a martyr for freedom.”

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Foley, 40, of Rochester, N.H., was taken prisoner in Syria in November 2012 while on assignment for the Global Post, an online news site. Sotloff, a Miami native, has been missing in Syria since August 2013. He wrote for the Christian Science Monitor, Time magazine and other publications.

Amid worldwide revulsion over Foley’s execution, British Prime Minister David Cameron said that the masked, black-clad Islamic State fighter who killed Foley appeared to be one of up to 400 Britons estimated to have joined the terrorist group.

“We have not identified the individual but it looks increasingly likely that it is a British citizen,” Cameron told reporters after breaking from a vacation to return to London to consult with his top aides on the Iraq crisis. “This is deeply shocking. We know that far too many British citizens have traveled to Iraq and traveled to Syria to take part in extremism.”

Before executing Foley, the fighter warned Obama that Sotloff would be killed unless U.S. airstrikes ended, speaking in fluent English with an accent that some experts reportedly associated with north London.