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 U.S.-made film 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.

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

U.S. President Barack Obama on Wednesday condemned the killing of the four Americans.

The president said in a statement that they exemplified America's “commitment to freedom, justice, and partnership with nations and people around the globe,” adding that they stood in “stark contrast to those who callously took their lives.”

Mr. Obama described Stevens as a “courageous” representative of the United States who had selflessly carried out his duties throughout the Libyan revolution.

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

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.”

Clinton said in an address at the State Department Wednesday, “we have new heroes to honor” and mourn.

She said Stevens “risked his life to stop a tyrant,” referring to Gadhafi, and then gave his life to build a better Libya.

Clinton said the United States will continue to work with the government and people of Libya, but pledged to bring those responsible for the deaths to justice.

Clinton 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.

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. The Associated Press reported that Stevens and his colleagues were killed when he went to the building to evacuate staff.

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.”

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 mobs were sparked by outrage over the film that U.S. media said was produced by Israeli-American Sam Bacile, who describes Islam as a “cancer,” and financed by expatriate members of Egypt's Coptic Christian minority group. Clips of the film in English and Arabic have recently been posted on YouTube.

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