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Libyan rebels fighting to topple Col Muammar Gaddafi have been edging west from the city of Misrata as they try to advance towards the capital, Tripoli.

Nato dropped leaflets on Zlitan, just west of Misrata, urging government troops to abandon their posts.

In Tripoli, fresh explosions took place as raids by the alliance resumed overnight.

Meanwhile, Canada has become the latest country to recognise Libya's National Transitional Council (NTC).

Correspondents say there has been a resurgence in fighting in Libya, following weeks of military stalemate.

At Zlitan, just west of the rebel-held city of Misrata and about 135km (85 miles) from Tripoli, Nato forces dropped leaflets bearing the Nato symbol, along with an image of an Apache attack helicopter and burning tanks.

An Arabic script reads: "There's no place to hide. If you continue to threaten civilians, you will face destruction."

Rebels say they have been told by Nato to hold the line between Misrata and Zlitan in advance of expected bombing runs.

Earlier, rockets damaged generators at an oil refinery near Misrata's port, disrupting fuel supply lines, Reuters news agency reported.

In Tripoli, two loud explosions sent plumes of smoke rising above the city late on Tuesday.

Before that, smoke was seen rising near Col Gaddafi's Bab al-Aziziya compound, though it was not clear exactly what had been targeted. Nato has frequently pounded the area in and around the sprawling compound.

The military alliance said it had struck targets such as a rocket launcher and an armoured vehicle mounted with anti-aircraft guns east of the capital.

There were reports of clashes in other parts of the country as well.

Omar Hussein, a spokesman for rebels in the western Nafousa mountains, said Col Gaddafi's forces were bombarding roads leading to the border crossing of Dahiba.

Rebels seized a number of towns in the area earlier this month, and Dahiba is a key supply route for them.

Witnesses told Reuters that pro-Gaddafi forces had fired Grad rockets over the border into Tunisia.

On the eastern front, a rebel commander told AFP news agency that 21 rebel fighters had been killed in clashes on Monday.

The latest strikes came amid concerns about the length of Nato's mission in Libya.

Gen Stephane Abrial, a senior Nato official, said coalition resources would become "critical" if the operation in Libya continued.

Nato took over the Libyan mission on 31 March.

South African President Jacob Zuma said the UN resolution that authorised the use of force to protect civilians in March was being abused for "regime change, political assassinations and foreign military occupation".

On Tuesday Kirsan Ilyumzhinov, the Russian head of the World Chess Federation, said Col Gaddafi had told him that he was ready to hold talks if Nato stopped its air strikes, but dismissed international efforts to get him to stand down.

The two played a game of chess in Tripoli on Sunday.

Mr Ilyumzhinov quoted the Libyan leader as saying: "I will not go anywhere. My relatives died here and I will also die in that land."

Libya condemned a visit by German Foreign Minister Guido Westerwelle to Benghazi, calling it a "blatant violation of national sovereignty and... international laws".

Canada and Germany are the latest countries to recognised the NTC, which is based in the eastern city, as the legitimate representative of the Libyan people.