US ambassador to Libya and three other embassy staff were killed as a mob armed with guns and rockets stormed the American consulate in the eastern Libyan city of Benghazi, angry over a film deemed offensive to Islam.

Christopher Stevens and three consulate staff were killed when the envoy went to the consulate as a frenzied mob stormed the premises and ran over the facility setting it ablaze, Libyan and US officials said.

The attack on the consulate came hours after protesters attacked the US Embassy in Cairo, tearing down the Star and Stripes and hoisting a black one that read, "There is no God but Allah and Mohammad is his messenger."

Media reports warned of more such protests erupting in Egypt as the ruling Muslim Brotherhood called for a nationwide protest against the film ridiculing Prophet Mohammad.

Describing the attack as "outrageous", President Barack Obama ordered stepping up of security in US diplomatic posts all over the globe.

"The United States condemns in the strongest terms this outrageous and shocking attack. We're working with the government of Libya to secure our diplomats," Obama said in a statement at Rose Garden of the White House, with the Secretary of State, Hillary Clinton, standing by his side.

"We will not waver in our commitment to see that justice is done for this terrible act. And make no mistake, justice will be done," Obama said.

Stevens was a career diplomat who spoke Arabic and French and had served two terms in Libya, including running the Benghazi office during the revolt against Muammar Gaddafi.

Libyan officials said Stevens was killed when he went to the consulate to evacuate the staff besieged by the frenzied mob.

Though, US officials did not give the cause of the death of the Ambassador, pan-Arabic news channel Al-Jazeera quoted Libyan Interior Ministry as saying that Stevens died of suffocation along with two US security guards who were accompanying him.

It said another consulate employee, whose nationality could not immediately be confirmed, was also killed.

Reports said hundreds of angry protesters brandishing assault weapons fired gunshots and attacked the US consulate with rocket propelled grenades in Benghazi as they clashed with the Libyan guards.

These were the first such assaults on US diplomatic facilities in Libya and Egypt, and came at a time when both nations are in turmoil following the ouster of their longtime leaders Gaddafi and Hosni Mubarak.

Condemning the outrage, US Secretary of State Hillary Clinton said she was heartbroken over the attacks and confirmed that a US diplomat was killed.

"As we work to secure our personnel and facilities, we have confirmed that one of our State Department officers was killed. We are heartbroken by this terrible loss. Our thoughts and prayers are with his family and those who have suffered in this attack," Clinton said in a statement.

"In light of the events of today, the United States government is working with partner countries around the world to protect our personnel, our missions, and American citizens worldwide," she added. Clinton called Libyan President to coordinate additional support to protect Americans in Libya.

"We are working to determine the precise motivations and methods of those who carried out this assault. Some have sought to justify this vicious behavior along with the protests that took place at our embassy in Cairo yesterday as a response to inflammatory material posted on the Internet," Clinton said.

"America's commitment to religious tolerance goes back to the very beginning of our nation. But let me be clear, there is no justification for this. None," she added.

Libya's General National Congress strongly condemned the incident and apologised for the killings.

"We present our apologies to the United States, the American people and the entire world for what happened," the GNC's president, Mohamed al-Megaryef, said in a statement.

UN Secretary General Ban Ki-moon and the 15-nation Security Council issued strong condemnations of the attack on the US consulate in Benghazi, asserting that there is no justification for such "brutal" violence and underlined the need to bring the perpetrators to justice.

Ban said he is "deeply saddened" by the attack against the US Consulate that resulted in the deaths of four Americans as well as Libyan employees of the consulate.

The protests against the film spread today, with demonstrations held against US embassies in Sudan, Tunisia and Morocco, though there was no report of violence.

The Vatican too condemned the "unacceptable violence" as well as "provocations" that led to it.