US President Barack Obama's anti-terror adviser, John Brennan, was in Libya on Wednesday to discuss investigations into the killing of the US ambassador and three other Americans last month, an official said.

Brennan was to meet Mohammed Megaryef, president of the Libyan national assembly, as well as foreign and interior ministry officials, said the Libyan official.

His visit comes almost one month after the September 11, 2012 attack on the US mission in the eastern Libyan city of Benghazi in which four Americans, including ambassador Chris Stevens, were killed.

The Obama administration has come under fire from Republican foes for the contradictory reports which have come out about the attack, amid allegations of security failures.