The American ambassador to Libya and three other US diplomats were killed overnight in an attack on the American consulate in Benghazi by Islamist militants.

Chris Stevens, who had completed several missions to Libya since 2007 and was appointed ambassador in March, was killed by a rocket-propelled grenade launched by gunmen from Islamist group, Ansar al-Sharia, according to a Reuters report.

The gunmen were protesting the release of a controversial film produced independently in the US that attacks Islam and denigrates Prophet Muhammad.

The consulate was set on fire in the attack, while video footage of a lifeless Caucasian man thought to be Stevens, later appeared on YouTube.

The identities of the three US diplomats killed in the attack had not been revealed at the time of press.

The attack in Benghazi followed similar protests against the film outside the US embassy in Cairo. Demonstrators scaled embassy walls and tore down the US flag before burning it.

The protests also came on the 11th anniversary of the September 11 terrorist attacks that killed almost 3,000 Americans.

“The United States deplores any intentional effort to denigrate the religious beliefs of others,” said US Secretary of State Hillary Clinton in the wake of the protests. “But let me be clear: There is never any justification for violent acts of this kind.”