Libyan government forces stepped up shellfire on the besieged western city of Misrata on Sunday. They pounded the port, which has been the rebel-held city's only lifeline.

A rebel spokesman said shells started landing at the port area as an aid ship was trying to unload. Meanwhile two other vessels bringing cargo to the city were forced by the shelling to wait offshore. But Libyan state television said the port was shelled to stop NATO from delivering weapons to the insurgents.

Reports said 12 people were killed, raising the two-day death toll to 29. On the other side of the city, rebels said they had opened up a new front, attacking pro-Gaddafi forces near the airport.

Meanwhile, artillery fire spilled into Tunisia from Libya on Sunday, and witnesses spoke of more intense fighting over a key supply route for rebels.