Islamist gunmen staged a military-style assault on the US consulate and a safe house refuge in Benghazi, eastern Libya on Tuesday.
US President Barack Obama vowed to "bring to justice" those responsible and the US military moved two navy destroyers towards the Libyan coast, in what a US official said was a move to give the administration flexibility for any future action against Libyan targets.
Obama said security was being increased at US diplomatic posts around the globe and on Thursday the US consulate in Berlin was partially evacuated after an employee fell ill on opening a suspicious envelope.
About 1,000 Bangladeshi Islamists tried to march on the US embassy in Dhaka after protests earlier in the week outside US missions in Tunisia, Sudan and Morocco.
The US military dispatched a Marine Corps anti-terrorist team to boost security in Libya, whose leader Muammar Gaddafi was ousted in a US-backed uprising last year.
The attack, which US officials said may have been planned in advance, came on the 11th anniversary of al Qaeda's attacks on the United States on Sept 11, 2001.
The attackers were part of a mob blaming America for a film they said insulted the Prophet Mohammad. Clips of the Innocence of Muslims, had been circulating on the internet for weeks before the protests erupted.
For many Muslims, any depiction of the Prophet is blasphemous and caricatures or other characterisations have in the past provoked protests all over the Muslim world.
An actress in the California production said the video as it appeared bore no resemblance to the original filming. She had not been aware it was about the Prophet Mohammad.
Among the assailants, Libyans identified units of a heavily armed local Islamist group, Ansar al-Sharia, which sympathises with al Qaeda and derides Libya's US-backed bid for democracy.
Former Libya militant commander Noman Benotman, now president of Britain's Quilliam think-tank, said Western officials were investigating a possible link with a paramilitary training camp about 100 miles (160km) south of the eastern Libyan town of Darnah.
US officials said there were suggestions members of al Qaeda's north-Africa based affiliate may have been involved.
Yemen, a key US ally, is home to al Qaeda in the Arabian Peninsula (AQAP), viewed by Washington as the most dangerous branch of the militant network established by Osama bin Laden.
The attacks could alter US attitudes towards the revolutions that toppled secularist authoritarian leaders in Egypt, Libya and Tunisia and brought Islamists to power.
The violence also could have an impact on the closely contested US presidential race ahead of the Nov. 6 election.
Republican Mitt Romney, Obama's challenger, criticised the president's response to the crisis.
He said the timing of a statement from the US embassy in Cairo denouncing "efforts by misguided individuals to hurt the religious feelings of Muslims" made Obama look weak as protesters were attacking US missions.
Obama's campaign accused Romney of trying to score political points at a time of national tragedy.
The attack raised questions about the future US diplomatic presence in Libya, relations between Washington and Tripoli, and the unstable security situation after Gaddafi's overthrow.
Stevens, a 52-year-old California-born diplomat who spent a career operating in perilous places, became the first American ambassador killed in an attack since Adolph Dubs, the US envoy to Afghanistan, died in a 1979 kidnapping attempt.
A Libyan doctor pronounced him dead of smoke inhalation.
US information technology specialist Sean Smith and two other Americans who have not yet been identified also were killed when a squad of US troops sent by helicopter from Tripoli to rescue the diplomats from the safe house came under mortar attack.
"It was supposed to be a secret place and we were surprised the armed groups knew about it," Libyan deputy interior minister Wanis al-Sharif said of the safe house.
Libyan leader Mohammed Magarief and Yemeni President Mansour Hadi both apologised to the United States over the attacks and Egypt's Islamist President Mohamed Mursi condemned them on television while also rejecting any "insult to the Prophet".
Many Muslim states focused their condemnation on the film and will be concerned about preventing a repeat of the fallout seen after publication in a Danish newspaper of cartoons of the Prophet Mohammad.
This touched off riots in the Middle East, Africa and Asia in 2006 in which at least 50 people died.
Afghan President Hamid Karzai called the making of the movie a "devilish act" but said he was certain those involved in its production were a very small minority.
The US embassy in Kabul appealed to Afghan leaders for help in "maintaining calm" and Afghanistan ordered the YouTube site shut down so Afghans would not be able to see the film.
How outrage against film unfolded
Clinton condemns anti-Islam video as disgusting
Afghanistan orders YouTube block over anti-Islam film
Saudi condemns anti-Islam film, attacks on US missions
Egypt's Mursi condemns embassy attack, protesters clash