Libyan government forces rained scores of rockets into residential districts of Misurata for a third consecutive day Saturday as ground troops tried to cut the city’s access to the port, its lifeline for food, water and medicine, rebel groups in the city said.

Civilians enduring a weeks-long assault that Western leaders have described as a “medieval siege” pleaded with NATO to intervene to prevent what they said was an impending massacre by forces loyal to Libyan leader Moammar Gaddafi.

“Our lifeline is the port, and he is going for it,” said Mohammed, a city council spokesman who for security reasons uses only his first name. “If he is able to do that, then we really are in trouble.”

“If a massacre occurs in Misurata, what will be NATO’s position?” added Mohammed, who spoke via Skype. “It is now or never. Either they intervene immediately and bring in ground troops to protect the port or we will all regret this.”

NATO’s mandate under U.N. resolution 1973 is to protect Libyan civilians, a mission that appears to be going unfulfilled in Misurata. The city, which lies 131 miles east of the capital, Tripoli, is the only one in western Libya under rebel control and a major strategic prize in the conflict.

Doctors reached through Skype said five people died in Misurata on Saturday, bringing the death toll there to 36 in the past three days alone and at least 276 since the siege began in late February.

Farther east, rebels said that four days of NATO airstrikes had helped them advance toward the strategic oil town of Brega, a city that has already changed hands half a dozen times.

But in Misurata, there was no respite, with NATO citing the difficulty of identifying clear targets within the city.

The fighting intensified Thursday when forces loyal to Gaddafi shelled Misurata’s port, forcing it to close for the day and interrupting the flow of humanitarian aid and the evacuation of civilians. Five Egyptians sheltering in a refugee camp for foreign workers were among the dead that day.

On Friday, there was evidence of cluster bombs being used by the government, according to Human Rights Watch, and further shelling and heavy fighting took place on the city’s main thoroughfare, Tripoli Street, and on the main road between the port and the city.

Rebels said government troops had been seen on foot in the city center Friday, an unusual sight as most usually stay in their tanks and armored cars.

On Saturday, government forces shelled a dairy factory and bakeries in the city, rebels said.

The Libyan government has strenuously denied using cluster bombs and has blamed civilian deaths on the “armed gangs” occupying Misurata, an assertion contradicted by UNICEF and foreign aid workers.

Every day, men, women and children are admitted to the hospitals and clinics of Misurata with head, chest and leg wounds from sniper fire and shrapnel, said Morten Rostrup of Doctors Without Borders who visited the city Friday to bring in supplies and help evacuate civilians.

Local doctors told Rostrup they were managing but could easily run out of supplies if casualties keep coming in, he said, adding that many patients were being discharged too early, to make room for others.

“If the port was cut off and there was no way to get supplies in, it would create a very dire humanitarian situation,” he said, speaking by phone from Tunis, where he had just arrived.

Misurata’s port is also a conduit for guns and ammunition sent to the rebels from their stronghold in Benghazi, 500 miles to the east.

Rostrup said thousands of foreign workers were sheltering in a makeshift camp by the port hoping to be evacuated. They had plastic sheets and blankets for shelter, not enough water and were facing an epidemic of gastroenteritis, he said.

Residents of Misurata say they are grateful for the help NATO has provided, but as the shelling continues, they say they need more.

“They are shelling a factory today that makes milk and dairy products for children,” said Aiman Abushahma, a doctor, who spoke via Skype. “We are at war, under siege, and it seems like no help is coming from NATO or anybody.”

“They are firing Grad rockets,” he added, referring to Russian truck-mounted multiple rocket launchers. “Surely they are big enough to be seen by NATO and to be destroyed?”

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