President Barack Obama has vowed the United States will "work with the Libyan government to bring to justice" those who killed US ambassador Chris Stevens and three other Americans in an attack on the consulate in Benghazi.
"Make no mistake, justice will be done," he said at the White House.
Mr Obama, who ordered an increase in security at US facilities overseas, said he "condemns in the strongest possible terms the outrageous and shocking" attack.
He spoke after Republican presidential challenger Mitt Romney condemned the attack, and criticised the administration for its initial response to a separate incident on Tuesday, the breach of the US embassy in Cairo.
The president spoke with secretary of state Hillary Clinton at his side.
Mr Stevens, 52, and three other Americans were killed when he and a group of embassy employees went to the consulate to try to evacuate staff as the building came under attack by a mob armed with guns and rocket propelled grenades.
The incidents in Benghazi and Cairo were reportedly the work of protesters angry over a film that ridiculed Islam's Prophet Mohammad.
In his remarks, Mr Obama said: "We reject all efforts to denigrate the religious beliefs of others, but there is absolutely no justification for this type of senseless violence, none."
The White House said Mr Obama was informed on Tuesday night that Mr Stevens was not accounted for and was notified on Wednesday morning that he had been killed.
In a written statement earlier, Mr Obama called Mr Stevens a "courageous and exemplary representative of the United States".