Libya has denounced as illegal Italian foreign Minister Franco Frattini's visit to the rebel stronghold city of Benghazi.

Libya "firmly condemns this illegal visit," a statement from the foreign ministry in Tripoli said, as reported by AFP.

Frattini's visit was "a flagrant violation of international norms and conventions and interference in the internal affairs of a sovereign and independent member of the UN," it added.

Frattini was visiting as Italy opened a consulate in Benghazi, reinforcing the rebel administration's diplomatic standing.