Nato warplanes have launched a daytime raid on the Libyan capital Tripoli.

A thick cloud of black smoke rose high into the air after a strike in the south of the city.

A series of explosions rumbled across what appeared to be other parts of the city as fighter jets could by heard flying overhead.

Fire engines raced through the streets, sirens blaring.

It was not clear what was hit or whether there were casualties. Friday is the main day of rest in Libya, with many people off work.

Nato has been ramping up the pressure on Muammar Gaddafi's regime. Though most air strikes happen under cover of darkness, daytime raids have grown more frequent.

The raids follow a barrage that struck multiple targets late on Thursday.

The fresh strikes blasted the capital as renewed diplomatic efforts to halt Libya's civil war appeared to be gaining momentum, though there are no signs a breakthrough is imminent.

On Thursday, Russia's envoy to Libya met senior government leaders in Tripoli, but not Gaddafi himself, in an effort to stop the fighting.

Last week, the envoy Mikhail Margelov visited the Libyan rebel stronghold of Benghazi and said that Gaddafi has lost his legitimacy. However, the envoy also said Nato air strikes are not a solution to Libya's violent stalemate.

Libyan Prime Minister al-Baghdadi al-Mahmoudi said the Libyan government has held a number of "preliminary meetings" with officials based in the eastern rebel-held city of Benghazi.

He said the talks took place abroad, including in Egypt, Tunisia and Norway, but he did not provide specifics.