Forces loyal to Muammar Gaddafi could retake Misrata within days unless Nato steps up its military intervention to assist the besieged rebels, according to an opposition spokesman inside the shell-battered city.

"It will be over in a few days, honest to God," said Mohamed, who asked for his full name to be withheld. The rebel fighters and civilians of the city were like "rats in a cage, that is to put it mildly", he said.

Nato's mandate was to protect civilians, "but every day there are scores of people dying. The human suffering goes on and on."

Mohamed, speaking from Misrata on Skype, claimed there had been no Nato air strikes on government positions, which are surrounding the city on three sides, for three days. "We are being hit by mortars, Grads, Katyushas, you name it – and there is no action."

Anas Rifta, a doctor inside Misrata, said there were acute shortages of baby formula, nappies, and renal, cardiovascular and cancer drugs, and medication to treat psychotic conditions. "If this situation sustains we will face a crisis. Even when we receive [fresh supplies], they are depleted in a small time."

There were still supplies of anaesthetic drugs, but they were finite, he said, and the city's clinics were overwhelmed with injured patients. "We make beds in the corridors and offices."

He said Misrata desperately needed aid and medical supplies, "but all [Gaddafi] is sending us is mortar bombs, cluster bombs and tank shells."

Electricity, water and basic foodstuffs are in short supply, according to residents. "Bread queues are very long," Donatella Rovera, a researcher for Amnesty International who is in the city, told the Guardian by phone.

There were also queues for fuel, not because of shortages but because "there are so few petrol stations open in safe areas", she said.

"The situation is really dire. There are huge problems with water and electricity." The city's sewage treatment plant and water reservoirs were in areas controlled by government forces, she said. "Coping with daily life is really hard."

Although supplies were arriving by boat, the quantities were inadequate to meet need, she said. There had been heavy shelling in the west of the city for the past three days. "A lot of families have left. They are crowding in with other families. There are people who have moved several times in three weeks."

Four civilians were killed on Monday and the death toll on Sunday was 25, Gemal Salem, a spokesman for the insurgents, told Reuters.

More than 1,000 people have died since the siege began seven weeks ago, according to Mohamed. The official hospital death toll was 304 until Sunday night, he said, "but a much greater number are pronounced dead at the scene and buried immediately". There were further numbers "missing, presumed dead. Gaddafi's forces kidnap the bodies and present them as people killed by Nato."

There was renewed shelling of the city on Monday – "the industrial area, the port area – the whole city" – and significant battles on Heavy Transport Road leading to the port, said Mohamed.

Although Misrata has been under siege for seven weeks, it has been the focus of international attention only for about a fortnight. The battle for the city has become emblematic of the civil war in Libya, and a symbol of Nato's failure so far to protect civilians caught up in it.

There have been chaotic and desperate scenes at the port – the only means of access to and from Misrata – in recent days as ships bringing aid and attempting to evacuate injured civilians and refugees struggle to dock under fire.

"One ship came in last night, and two more are due today," said Mohamed. "I was there last night. The captain said he had only one hour to offload the aid and load refugees. It was chaotic. The shelling puts great strain on those in the port."

The International Organisation for Migration (IOM) said almost 1,000 people trapped in Misrata had been evacuated by boat and were due to reach Benghazi later on Monday. At least 4,000 migrant workers were waiting to be evacuated, the IOM said.

Some of those who have managed to flee described terror and wretched conditions in the city. "In Misrata, people are just waiting for their time to die. You hear the bombs and you think 'It's me next,'" said Wajdi Salah, 26, who arrived in Benghazi on a tugboat last week. Most civilians were now crowded into three areas of the city, all near the sea, with up to a dozen families crammed into a single house.

Some captured civilians had been used as human shields by Gaddafi's forces, said Abubakar Faraj from a hospital bed in Benghazi. The shelling was indiscriminate. "At night they [government forces] wear thermal goggles, and they shoot everything that moves. Even dogs and cats."

Attempts by humanitarian organisations to reach the besieged population are mounting. The UN has signed an agreement with the Libyan government which the Libyan authorities say includes access to Misrata, although the UN has not confirmed this.

The International Committee of the Red Cross has also been permitted to visit Misrata for an assessment but has made no public statement on its findings.

Mohamed rejected the Libyan government's claims that al-Qaida was driving the rebellion. "I know lawyers, engineers, even a professional football player who have taken up arms. It is the whole people. The whole of Libya has been oppressed by [Gaddafi] for 42 years. Nobody thinks he is even human, let alone loves him."

The rebel opposition felt "disappointed and let down by Nato", he said. "We ask the US to take the lead again. Since Nato took over there has been nothing but carnage and destruction. Nato is toothless without the US. They are selling us to the tyrant. Please, please, please tell the world of our disappointment with Nato."