US Assistant Secretary of State Jeffrey Feltman has said Libya’s rebels have accepted an invitation to open a representative office in Washington.
Mr Feltman is the most senior US diplomat to visit the rebels in their de facto capital Benghazi.
The US has insisted that Libyan leader Col Muammar Gaddafi step down, but has not given the rebels full recognition.
The visit came as Nato planes launched a series of air attacks on Libya’s capital, Tripoli, the heaviest so far.
Libyan officials say three people were killed and dozens injured in an attack on the barracks of the popular guard.
Nato says it bombed a vehicle depot next to Col Gaddafi’s Bab al-Aziziya compound which had been used by his forces in attacks on civilians. However, the Libyan government described it as a reserve military base and said the casualties were civilians.