WASHINGTON -- The United States military conducted an air and ground operation in Syria this summer which failed to rescue several American hostages held by the Islamic State, in the very first such mission by US forces since the Syrian war began over three years ago."This operation involved air and ground components and was focused on a particular captor network within ISIL," Pentagon press secretary Rear Admiral John Kirby said on Wednesday. "Unfortunately, the mission was not successful because the hostages were not present at the targeted location."The Pentagon did not specify the number of hostages suspected to be held in Syria, where the operation occurred specifically or how many US military personnel were involved.US troops were fired upon with significant force by Islamist militants during the operation, according to officials.The White House intended to keep the attempted rescue mission secret, one spokesman said on Wednesday night. Only when details of the operation were leaked by administration officials did the White House decide to comment.“We never intended to disclose this operation," National Security Council spokeswoman Caitlin Hayden said, responding to multiple media requests. "An overriding concern for the safety of the hostages and for operational security made it imperative that we preserve as much secrecy as possible."We only went public today 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," Hayden continued.General John R. Allen (Ret.) lauded Obama for his actions against the Islamic State on Wednesday night— but called on the president to broaden the mission, and to destroy the group "quickly" before its terror can further metastasize."Make no mistake," Allen wrote, in an op-ed featured on DefenseOne, "the abomination of [the Islamic State] is a clear and present danger to the US." Allen, a retired four-star general who ran operations for the US in Afghanistan, was nominated in 2013 for the position of NATO's Supreme Allied Commander in Europe."The only question really is whether the US and its allies and partners will act decisively now," he continued," while they can still shape events to destroy [Islamic State]."The revered general's comment was published a day after the Islamic State released a gruesome video online, which captured the graphic beheading of James Foley, an American journalist captured in Syria in 2012.Foley's killer, who speaks in the video with a London accent, is understood to be a British citizen by the UK intelligence community, according to Prime Minister David Cameron, who returned from his summer holiday for crisis meetings on the matter.Cameron called the act nothing short of "murder without any justification," and said that the perpetrator was "likely" a British national.British media outlets circulated rumors of the murderer's origins, citing sources that identified him as "John," a member of a group of extremist Arab Londoners. Independently, government officials confirm that close to a quarter of roughly 2,000 foreign nationals fighting in Syria are British.The Pentagon continued to push forward with strikes against ISIS targets throughout northern Iraq. Over a dozen strikes were conducted after ISIS released the video to the public. A second US journalist, Steven Sotloff, would be killed next if the US continued strikes, the group stated."First of all, there is no justification for these kind of barbaric acts, period," State Department deputy spokeswoman Marie Harf told reporters on Wednesday. "Second, we don’t make concessions to terrorists," she continued. "The president was clear we’re going to keep doing what we’re doing. And I would also note that, as I said earlier, ISIL has been willing to kill and rape and enslave anyone who gets in their way, regardless of what country they’re from, regardless of the policies of that country."Harf could not confirm that either Obama or US Secretary of State John Kerry had watched the video.In a statement from Massachusetts earlier in the day, US President Barack Obama sternly condemned the videotaped murder of Foley. Foley was captured in Syria in 2012, working as a freelancer for news Web site GlobalPost. The Islamic State holds court in its nominal capital, al-Raqqa, in eastern Syria.Obama administration officials say the president reserves all options to protect Americans under threat in the region. But the president has ruled out "boots on the ground" to fight the Islamic State, either in Iraq or Syria, where the group has control over territory. On Wednesday, Obama called the Islamic State a "cancer" that the world must prevent from spreading.In a prepared statement, Lisa Monaco, Assistant to the President on Homeland Security and Counterterrorism, said that the White House would be unable to confirm further details of the mission for the protection of military personnel in the region."Earlier this summer the president authorized an operation to attempt the rescue of American citizens who were kidnapped and held by ISIL against their will in Syria," Monaco 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 in ISIL custody."The US 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," she continued.