US President Barack Obama has pledged to bring to justice those who attacked the US consulate in the eastern Libyan city of Benghazi.
The Tuesday night assault, which came amid a protest and reportedly involved RPGs and grenades, left Ambassador Christopher Stevens and three other US personnel dead, as well as five others wounded.
US officials confirmed to Al Jazeera that a special unit of roughly 50 members of the Marine Corps had been dispatched to Libya to reinforce the troops guarding diplomats there, as two warships headed to the Libyan coast.
The Marines' Fleet Anti-Terrorism Security Team (FAST) detachment is specially trained to deploy rapidly to protect government workers overseas.
Speaking at the White House early on Wednesday, Obama pledged that the incident would not "break the bonds" between the two countries.
He condemned the attack, calling it "outrageous and shocking", and said he had ordered increased security at US diplomatic posts around the world.
"No act of violence will shake the resolve of the United States of America," he warned later on Wednesday at the start of a campaign event in Las Vegas, Nevada.
"We will not be deterred - we will keep going, we will keep going because the world needs us," the president said. "We are the one indispensable power in the world."
The attack in Benghazi and protests in Cairo, Egypt, were apparently prompted by the video trailer for a small-budget film said to be produced by man claiming to be an "Israeli-American real-estate developer in California" that gives an insulting depiction of the Prophet Muhammad.
But the identity of the director "Sam Bacile", and the status of the actual film, have been called into question.
For his part, Libya's interim president, Mohammed el-Megarif, apologised for what he called the "cowardly" assault on the consulate, which also killed several Libyan security guards in the eastern city.
He offered his condolences to the US and also pledged to bring the culprits to justice and maintain his country's close relations with Washington.
"We extend our apology to America, the American people and the whole world," Megarif said.
Obama's remarks followed a public statement by Hillary Clinton, US secretary of state, who pointedly drew a distinction between the crowd that attacked the consulate and other Libyans.
"Indeed, I asked myself, how could this happen, how could this happen in a country we helped liberate, in a city we helped save from destruction," she said.
"This question reflects just how complicated and at times how confounding the world can be, but we must be clear-eyed even in our grief. This was an attack by a small and savage group, not the people or the government of Libya."
Libyan political commentator Waheed Burshan discusses US ambassador's killing in Benghazi attack
Clinton and Obama stressed the country's respect for all faiths and history of religious tolerance but said the attack was "senseless" and unjustifiable.
"Violence like this is no way to honour religion or faith," Clinton said.
Tuesday's consulate attack followed a larger protest at the US embassy in Cairo, where thousands shouted religious slogans and tore apart the US flag.
The campaign of Republican presidential nominee Mitt Romney criticised the US administration's response to events in Cairo in the hours before Clinton and Obama made their statements and before Stevens' death was confirmed.
Romney's campaign said Obama's initial response was "disgraceful" and "was not to condemn attacks on our diplomatic missions, but to sympathise with those who waged the attacks".
The US embassy in Cairo issued a statement earlier on Tuesday condemning "the continuing efforts by misguided individuals to hurt the religious feelings of Muslims, as we condemn efforts to offend believers of all religions".
That statement was released at 12:22pm Cairo time, about four hours before protesters gathered outside the US mission there, and six hours before protesters breached the security wall.
Obama's campaign fired back, with a spokesman saying he was "shocked that, at a time when the United States of America is confronting the tragic death of one of our diplomatic officers in Libya, Governor Romney would choose to launch a political attack".
A UN official condemned the attack as "horrific and tragic", and Libya's UN ambassador said an investigation was under way.
Ibrahim Dabbashi told the UN Security Council that authorities would bring those responsible to justice.
"This attack in no way serves the interests of the people or the Libyan authorities and cannot be considered as a defence of Islam," he said. "This attack gravely damages the image of Islam."
The Russian foreign ministry also issued a sympathetic statement, saying it considered attacks on diplomats to be "manifestations of terrorism".
"Moscow views the events in Cairo and particularly in Benghazi, which led to the tragic deaths of four American diplomats, with deep concern," the statement said.
"We decisively condemn all attacks on foreign diplomatic representations and their employees as manifestations of terrorism that can have no justification."