| ANDREWS AIR FORCE BASE, Maryland
ANDREWS AIR FORCE BASE, Maryland President Barack Obama vowed on Friday that U.S. resolve would be unshaken by violent anti-American protests sweeping the Muslim world as he honored the return of the bodies of the U.S. ambassador and three other Americans killed in an attack in Libya this week.
Leading a somber ceremony with the flag-draped caskets of the Libya dead laid out beside him, Obama pledged to "bring to justice the ones who took them from us" and to hold foreign governments responsible for safeguarding U.S. diplomatic installations now under threat.
"The United States will never retreat from the world," Obama told an audience of grieving family members, diplomats and dignitaries inside a vast aircraft hangar at Andrews Air Force Base outside Washington.
Obama spoke as fury about a film that insults the Prophet Mohammad tore across the Middle East and other parts of the Muslim world after weekly prayers on Friday, with protesters attacking U.S. embassies and burning American flags as the Pentagon rushed to bolster security at U.S. missions.
The Pentagon is sending a platoon of Marines to bolster security at the U.S. Embassy in Khartoum, Sudan, following similar reinforcements sent to Libya and Yemen, a U.S. official said.
The obscure California-made film triggered an attack on the U.S. consulate in the Libyan city of Benghazi that killed U.S. Ambassador Christopher Stevens and three other Americans on Tuesday, the anniversary of the September 11, 2001 al Qaeda attacks on the United States.
The Libya attack and rapid spread of anti-U.S. violence to other Muslim countries have raised questions about Obama's handling of the Arab Spring - still-unfolding revolutions he has cautiously sought to nurture - even as he seeks re-election in November in a closely contested race.
The wave of attacks has reverberated in the presidential campaign, with Republican challenger Mitt Romney and his allies seizing the chance to reassert accusations that Obama has weakened U.S. global leadership.
Romney, who has been accused by his Democratic opponents of trying to exploit a national tragedy for electoral gain, paused in his campaigning in Ohio to watch the ceremony on television and then led a moment of silence for the slain Americans.
At a breakfast fundraiser earlier in New York, however, he suggested Obama's foreign policy lacked "clarity of purpose."
Obama's latest foreign policy crisis comes less than two months before the U.S. presidential election.
He acknowledged "these are difficult days" but expressed firm resolve to persist in his Middle East policies. "We will stand fast against the violence on our diplomatic missions," he the crowd of more than 200 mourners.
"We will continue to do everything in our power to protect Americans serving overseas, whether that means increasing security at our diplomatic posts, working with host countries which have an obligation to provide security and making it clear that justice will come to those who harm Americans," he said.
The State Department said some 65 U.S. diplomatic missions around the world had put out security or emergency messages for U.S. citizens as the protests expanded over the past two days.
Spokeswoman Victoria Nuland said no decisions had been taken on reducing embassy staff or their dependents in the affected countries.
The ceremony marked the poignant homecoming of the bodies of Stevens, State Department information management officer Sean Smith and security personnel Tyrone Woods and Glen Doherty, both former Navy SEALS.
Some U.S. officials believe the deadly attack could have been plotted in advance, but White House spokesman Jay Carney said on Friday he had no information suggesting the assault on the Benghazi consulate was "pre-planned."
Secretary of State Hillary Clinton, appearing to struggle to control her emotions, repeated that the U.S. government had nothing to do with the making of the inflammatory film, and she insisted the violent response was "totally unacceptable."
She said countries like Egypt, Tunisia, Libya and Yemen, which had cast off authoritarian rulers, "did not trade the tyranny of a dictator for the tyranny of a mob."
Echoing Obama's sentiments, the nation's top diplomat said: "We will wipe away our tears, stiffen our spines and face the future undaunted."
Also in attendance were Vice President Joe Biden, Defense Secretary Leon Panetta and Colin Powell, who served as secretary of state under Obama's predecessor, George W. Bush.
At the start of the ceremony, Marine pall bearers hoisted the caskets one by one from a military cargo plane and carried them into the hangar while a military band played somber music. As Obama spoke, he was flanked by four hearses with a large American flag hanging from the rafters behind him.
Stevens, 52, had spent a career operating in perilous places, mostly in the Arab world, and became the first American ambassador killed in an attack since Adolph Dubs, the U.S. envoy to Afghanistan, died in a 1979 kidnapping attempt.
The bodies of the four Americans were then flown to a military mortuary at an air base in Dover, Delaware.
(Additional reporting by Patricia Zengerle, Lisa Lambert, Steve Holland, Phil Stewart and Andrew Quinn. Writing by Matt Spetalnick; Editing by Todd Eastham)