The United States ambassador to Libya and three embassy staff were killed after a mob stormed the U.S. consulate in the eastern city Benghazi late Tuesday.

Demonstrators reportedly angered over an amateur film allegedly made in the U.S. that mocks Islam's Prophet Muhammad shot at and set fire to the U.S. consulate in Benghazi.

Ambassador J. Christopher Stevens is the first American envoy killed abroad in more than 20 years. He was a career U.S. foreign service officer and one of the most experienced U.S. envoys in the region. He had taken his post in the capital, Tripoli, in May.

U.S. President Barack Obama on Wednesday condemned the killing of the four Americans and vowed to hunt down and bring justice to the attackers.

Speaking at the White House Rose Garden with U.S. Secretary of State Hillary Clinton by his side, Mr. Obama said he is ordering increased security at U.S. diplomatic posts around the world. U.S. officials say some 50 Marines have been dispatched to Libya to boost security at U.S. diplomatic facilities.

After his address, Mr. Obama went to the State Department to console diplomatic staff.

Stevens was widely admired by the Libyan rebels for his support of their uprising that overthrew Libyan strongman Moammar Gadhafi. Secretary of State Clinton said Stevens “risked his own life to lend the Libyan people a helping hand to build the foundation for a new, free nation.”

She said the relationship between the U.S. and Libya will not be “another casualty” of the attack, and the U.S. will not turn its back on the Libyan transition to a free and democratic nation.

The U.S. State Department reported that U.S. Foreign Service Information Management Officer Sean Smith was also killed. Clinton said the department was notifying family members of the other two individuals.

Libyan Deputy Prime Minister Mustafa Abu Shagour has condemned the killing of the four U.S. diplomats, calling it a cowardly act.

Libyan Deputy Interior Minister Wanis al-Sharif told reporters that an armed group attacked the consulate premises in an “almost suicidal” mission. He said the U.S. consulate is at “fault” for not taking adequate precautions. Further details of the incident were unclear.

Earlier reports said several dozen gunmen from the Islamist group Ansar al Sharia attacked the U.S. consulate with automatic rifles and rocket-propelled grenades, then set it on fire.

Clinton said some have sought to justify “this vicious behavior” as a response to inflammatory material posted on the Internet. She said the U.S. deplores any intentional effort to denigrate the religious beliefs of others, but rejects any “justification for violent acts of this kind.”

Tunisian police fired tear gas Wednesday to disperse hundreds of demonstrators at the U.S. Embassy in the capital, Tunis. The protesters were condemning a film that media say was financed by expatriate members of Egypt's Coptic Christian minority group and was allegedly produced by Israeli-American Sam Bacile, who describes Islam as a “cancer.” Clips of the film in English and Arabic have recently been posted on YouTube.

In Egypt Tuesday, protesters scaled the walls of the U.S. embassy in Cairo, tore up an American flag and replaced it with an Islamic banner. The demonstrators there – mainly ultraconservative Islamists – continued their protest action through the early hours of Wednesday.

The protests coincided with the 11th anniversary of the September 11 terrorist attacks in the United States.