The premises of the US diplomatic mission in Libya's eastern city of Benghazi came under attack during the night but there were no casualties, embassy and security sources said on Wednesday.
"There was an attack late last night on the United States office in Benghazi," a US embassy official said, adding that only the gate was damaged and that no one was hurt.
He said home-made bomb had been used in the attack on the office, which was set up after the 2011 uprising against Muammar Gaddafi and has been kept open to support the democratic transition.
"The US deplores the attack on its diplomatic mission in Benghazi," he said, adding that a request had been made to Libyan authorities to step up security around US facilities in the North African nation.
The American official said there was no claim of responsibility.
But a security official in Benghazi said the attack was claimed by the Prisoner Omar Abdelrahman Group, which had left a letter "threatening American interests" in Libya.
On May 22, the same group claimed an attack on the offices of the International Committee of the Red Cross.
The latest attack comes hot at the heels on the death of Abu Yahya al-Libi, a master Al-Qaeda propagandist and a Libyan national targeted by a US drone strike in Pakistan.
The ruling National Transitional Council's spokesman said the authorities were trying to get more information on the incident and condemned the attack.
"We condemn these actions," said Mohammed al-Harizi.