The Libyan government has promised to allow the United Nations to open a humanitarian corridor to the besieged western city of Misurata to provide aid, food and medicine to civilians and safe passage for people to leave, after weeks of attacks that have left an estimated 1,000 people dead.

Residents and doctors in the rebel-held portion of the coastal city said 23 people were killed in heavy tank, mortar, rocket and sniper fire from government forces Sunday and that another five died Monday.

More than 300 men, women and children have been registered dead in the hospitals of the port city about 130 miles east of the capital, Tripoli. “But a greater number died in situ and were buried directly,” said Mohammed, a spokesman for the city council who prefers not to give his family name for safety reasons. He estimated that 1,000 people have died in total since the siege began in late February, but he said some doctors believe the death toll is higher.

U.N. Secretary General Ban Ki-moon appealed to Libyan forces Monday to hold their fire. “Considering the magnitude of this crisis, and as this fighting is still continuing, it is absolutely necessary that Libyan authorities stop the fighting, stop killing people,” he told a news conference in Budapest.

He said the United Nations would open a “humanitarian presence” in Tripoli and would try to expand operations with the help of the International Committee of the Red Cross and others. But Valerie Amos, the U.N. humanitarian chief, said she had received assurances from government authorities in Tripoli that U.N. officials would be allowed into Misurata itself.

“What I would like to do is get access to Misurata, not just from the sea, but also from the road,” Amos told reporters in Benghazi, according to the Associated Press. “We have very little sense of what is going on across the city.”

Moussa Ibrahim, a spokesman for the government of Moammar Gaddafi, said that under an agreement signed with the United Nations on Sunday, food, aid and medicine would be allowed into Misurata and civilians would be allowed to leave.

International aid agencies would also be given access to Misurata, and authorities would ensure that electricity, water and other services would be restored, he said. “It’s a very positive step.”

The Red Cross said it was also working with the Libyan government “to access people in need in and around Misurata.” But it said Libyan government reports that its team came under rebel fire at the weekend were false, and may have resulted from a “misunderstanding.”

In the city, though, residents who have faced barrages of government shelling for weeks were skeptical of the agreement. “All he [Gaddafi] is sending us is mortar bombs, cluster bombs and tank shells,” said Anas Rifta, a doctor speaking by Skype.

Rifta said baby formula and diapers have run out, as well as drugs to treat chronic life-threatening conditions such as cancer, heart disease and renal problems.

Among the residents, anger was mounting day-by-day with NATO for failing to protect them against Gaddafi’s barrages.

“We are officially let down and disappointed by NATO,” Mohammed said. He said there were no apparent airstrikes in the area in the past three days, allowing Gaddafi’s forces to intensify their shelling of the port, residential and industrial areas of the city.

“What is the mandate of NATO? It is protection of civilians, but civilians are dying in Misurata,” he said. If they cannot do it, they should say they cannot do it. NATO is toothless and powerless without the U.S.”

NATO has cited difficulties identifying targets on the ground, with patchy intelligence and government forces doing their best to camouflage their presence. It said its planes attacked four air defense radars near Misurata on Sunday, but it made no mention of striking the tanks, artillery emplacements or rocket launchers that are pounding the city.

Rebels say Gaddafi appears to be trying to cut the city off from the port, currently its only source of food, water and medicine. And aid workers warn of a very dire humanitarian situation for hundreds of thousands of trapped civilians if government forces succeed.

But Saif al-Islam Gaddafi, the influential second son of the Libyan leader, said the port was also being used to bring in a steady flow of arms, ammunition and “terrorists.”

“You want the Libyan government to sit and wait every day for the terrorists to get stronger?” he asked in an interview with The Washington Post. “The army was in dialogue and in negotiation with those people for one month, trying to convince them to lay down arms and go back home. One month, we failed. And then, they used the time to fortify their site. So you want us to repeat the same mistake again? Of course not.”

Nearly 1,000 people, most of them foreign migrant workers, were evacuated from Misurata on Monday. But the International Organization for Migration said conditions remain desperate for about 4,000 left behind and that it needs funding for a bigger boat to evacuate them.

“We wanted to be able to take on more people, but it was not possible,” Jeremy Haslam, who heads the group's boat rescue, told the Associated Press. “Although the exchange of fire subsided while we were boarding with an eerie silence at one point, we had a very limited time to get the migrants and Libyans on board the ship and then leave."

Correspondent Edward Cody in Paris contributed to this report.