By Fredrik Dahl and Souhail Karam

TUNIS/RABAT (Reuters) - Forces loyal to Muammar Gaddafi renewed the bombardment of Misrata Tuesday, causing a number of casualties, an Amnesty International researcher in the besieged Libyan city said.

Libya's third-largest city, the insurgents' last major stronghold in the west of the country -- has been under siege by government troops for more than seven weeks.

Rebels and residents say pro-Gaddafi forces have pounded Misrata heavily in recent days, firing rockets and mortars at insurgent positions and also hitting residential areas.

"They were shelling very close by, in the area slightly to the northwest of the center. I just left the hospital, there were casualties coming in," Amnesty's Donatella Rovera, who came to Misrata late last week, said by phone.

"These are the areas which are, for now, in the hands of the opposition and they are being shelled by Gaddafi forces," she told Reuters. "The city center is the front line."

Rebel spokesmen, citing hospital records, say hundreds of people have been killed in Misrata. They say insurgents have made gains on the ground in Misrata despite the shelling.

Aid groups say conditions are worsening in the city of 300,000, with a lack of food, medicines and other basic items.

International humanitarian organizations have started evacuating trapped civilians by boat from its rebel-held port.

"There is no electricity. The town is functioning on generators," Rovera said. "The supply of water has now been cut off for weeks. They've gone back to using old wells."

In Geneva, the U.N. Children's Fund (UNICEF) said at least 20 children have been killed in weeks of fighting in Misrata.

Libyan officials say they are fighting militia with ties to al Qaeda bent on destroying the country and deny government troops are shelling Misrata and its civilians.

Libyan forces are in defensive positions and striking back at pockets of armed gangs attacking them, Deputy Foreign Minister Khaled Kaim told reporters in Tripoli.

GUERRILLA TACTICS

Unlike eastern Libya, where rebels control many coastal cities, most of its west remains firmly under Gaddafi's control.

A rebel spokesman in the western city of Zawiyah, which insurgents held for several weeks after an anti-Gaddafi uprising erupted in mid-February, said they were now resorting to guerrilla tactics there after the government reasserted control.

"Groups, each comprising between 15 and 20 rebels, keep ambushing Gaddafi forces ... We have managed to kill dozens of them," the rebel who called himself Mohamed said by phone.

The rebels in the coastal town about 50 km (30 miles) west of Tripoli were defeated on March 10 after fierce battles.

"The Gaddafi forces have deployed checkpoints every 200 meters. They are heavily armed," Mohamed said.

"Since they took control of the city, they have arrested thousands, mostly young men on the slightest suspicion of sympathy for the rebels," he said.

Last Sunday, he said rebels on a pick-up truck fitted with a heavy machine-gun attacked a government forces headquarters in the city, killing 20. It was not possible to independently verify the claim.

"The city's suffering is beyond description," Mohamed said. "There is shortage in fuel, food and medicine."

(Additional reporting by Mussab Al-Khairalla in Tripoli; Editing by Angus MacSwan)