Hundreds of civilians are being killed or seriously injured in artillery and gun attacks as the Sri Lankan army attempts to finish off the last Tamil Tiger rebels trapped in a shrinking pocket of land.

The Observer was refused access to the Putumattalan field hospital by the military, but a doctor, Thangamutha Sathiyamorthy, sent this picture of injured Tamils awaiting treatment. (Photograph: Thangamutha Sathiyamorthy) Injured civilians lucky enough to get out have told of carnage in this so-called "no-fire zone" - a 17 sq km strip of coast where the Tigers are penned in with their backs to the sea.

Horrific stories of limbs ripped off by shellfire and bodies buried where they fell are emerging, despite the government's efforts to hide the scale of the killing by confining the injured to hospitals in a military area around the government-declared no-fire zone, from which the media are strictly excluded.

The casualties' graphic accounts of a fierce onslaught on the no-fire zone, supported by the evidence of their severe wounds, have been reported by doctors who have treated them at a field hospital at Pulmoddai, inside the military area, where thousands of evacuees have been taken by ship. According to the senior doctor handling the casualties for the Sri Lankan government as they arrive at Pulmoddai, shells are falling among the tightly packed tents and shelters that are home to tens of thousands of civilians, killing and wounding dozens every day.

"Most of the people have shell blast injuries and gunshot injuries. Some people have lost their limbs, other people have lost other parts of the body, some people have wounds in the abdomen, some in the chest," said Gnana Gunalan, a doctor who treats the flood of casualties as they arrive by Red Cross ship.

Gunalan, chairman of the local Sri Lankan Red Cross, said: "All these people are very badly traumatised. Some have lost all their loved ones and come here alone, one boy losing both legs. One girl came who had lost her husband and children and everybody."

The doctor said the accounts of the evacuees appeared to support previous claims from doctors in the no-fire zone that the shelling had not come from Tamil Tiger positions in the zone. The Sri Lankan government has vehemently denied firing into the zone, but it is not possible to verify the claims.

Gunalan - who is based in the town of Trincomalee, surrounded by heavy Sri Lankan army security - said that the field hospital at Pulmoddai had treated 1,468 casualties among the 5,456 people evacuated by sea from the no-fire zone in the last month. Doctors say most of those killed have been buried near where they died and there has been no attempt to bring out the bodies.

Determined to resist international pressure to stop the fighting before it has finished off the hardcore rump of the Tamil Tigers cornered by the military, the Sri Lankan government has kept casualties away from the eyes of the world. This weekend the government rejected an appeal by the UN to give civilians more time to leave the no-fire zone.

And last night the Sri Lankan military sources said 2,857 civilians had broken through Tamil Tiger lines and made their way to safety during the day. They added that 5,000 people had tried to escape and had come under fire from the rebels. But it was not possible to verify the reports because the military has denied access to the area surrounding the no fire zone.

Until last month the government allowed civilians injured in the no-fire zone to be taken to the larger hospital in Trincomalee, but then decreed that they must be kept inside the military area.

At Pulmoddai the most serious injuries are stabilised by a team of Indian doctors working in temporary metal huts. By Friday, 26 had died at the Pulmoddai field hospital. It is not possible to verify the doctors' accounts, because neither side will allow access to the no-fire zone. The military has permitted international media access on occasional day trips to the surrounding military area only. The Observer was refused entry to the hospital, turned back at a checkpoint on the edge of Pulmoddai, and refused access to the camps where those who have escaped the fighting are being held.

Pulmoddai is two and a half hours' drive north of Trincomalee, a journey involving countless military checkpoints. Soldiers are everywhere, in bunkers or standing beneath trees watching the road. No one can move without permission from the Defence Ministry.

However, reports continue to get through. Two regional health directors, defying government instructions, have described at length the extent of the unfolding humanitarian disaster.

One, Thangamutha Sathiyamorthy, told the Observer on Friday that civilians were still being killed and injured by shelling inside the zone. He also said there had been a number of attacks by helicopters. He said the previous day, five people had died in the hospital from their wounds and a child of 13 had perished from the effects of diarrhoea.

He said many people had dug shelters in the sand to try to escape the shelling. "The fighting is continuing. Shells are falling. But these people have no alternative. They cannot move. Most of the injuries we are treating are from shells and bullets. Today we received 58 injured civilians, including 16 children."

Most of the Tamil Tiger fighters were on the front line, he said, but some were moving among the civilians, visiting family members or moving casualties. He said Tamil Tiger police were still operating, attempting to control the crowds.

The Sri Lankan health secretary, Athula Kahandaliyanage, last week accused Sathiyamorthy and another doctor of spreading "malicious propaganda", claiming their accounts "cannot be credible, since these officials are operating under duress and the dictates" of the Tamil Tigers.

The Health Ministry says both doctors will face disciplinary action. But Sathiyamorthy denied he was under pressure from the LTTE. "We are telling the truth. The government has strongly asked us not to tell the truth, but we must."