Libya has apologised to visiting US deputy secretary of state William Burns for an attack on the US consulate in Benghazi in which American ambassador Christopher Stevens died.
Burns was holding talks in Tripoli with Libyan leaders on Thursday, including new Prime Minister Mustafa Abu Shagour and Mohammed al-Magarief, head of the national congress, following last week's assault, in which three other Americans were also killed.
He was also due to attend a ceremony commemorating Stevens.
Ashour Bin Khayyal, Libya's foreign minister, apologised for the violence on Tripoli's behalf, praising Stevens as a "friend of Libya", a foreign ministry official said.
The four Americans died when gunmen attacked the consulate and a safe house in the eastern city of Benghazi. The attackers were among a crowd protesting against a privately financed video made in California that mocks Islam and the Prophet Muhammad.
Matthew Olsen, director of the US government's National Counterterrorism Center, on Wednesday called the assault a "terrorist attack" and said officials were looking at whether those involved had links to al-Qaeda, particularly its North African affiliate, al-Qaeda in the Islamic Maghreb.
The Libyan foreign ministry official said Burns and Bin Khayyal had discussed US involvement in the investigation, as well as broader security and economic co-operation.
Magarief, who apologised last week "to the United States, the people and the whole world" for the Benghazi attack, also agreed in a telephone call with US President Barack Obama that their countries would work together to investigate it.
Meanwhile, the head of a committee tasked with finding posts for militia fighters in the police in eastern Libya said on Thursday he had quit, becoming the third senior security figure sidelined a week after a deadly attack on the US consulate in Benghazi.
Fozi al-Gaddafi, who is not related to ousted dictator Muammar Gaddafi, told the Reuters news agency he had resigned as eastern Libya head of the Security Committee in protest because recruits were not being adequately paid or supplied.
His deputy was acting in the post, he said.
The government in Tripoli announced this week that it was sacking the deputy interior minister for the east and the police chief of Benghazi, but both men have refused to step aside.
The man named to take on both jobs, Salah Doghman, told the Reuters news agency on Wednesday he asked the government to send troops if necessary to install him in his new job.