Sri Lankan armed forces were claiming "total victory" as they launched a final assault on the cornered Tamil Tigers in the north-east of the country, defying international pleas for a halt to the fighting and warnings from the United Nations of a bloodbath.

A Sri Lankan army photograph showing ethnic Tamil civilians on the run from the fighting. Photograph: Handout/Sri Lankan army With the army seemingly determined to wipe out the last remnants of the rebels, at least 30,000 civilians - possibly as many as 80,000 - remained trapped and unaccounted for inside the single square mile of territory still held by the Liberation Tigers of Tamil Eelam.

Unconfirmed reports suggeste that Tamil leaders were preparing to kill themselves rather than face capture. Some cadres are known to carry a cyanide capsule with them at all times.

Tamil sources in the UK said they were unaware of the reports and all contact with those left inside the "no-fire zone", the government-designated haven where non-combatants were supposed to be able to escape the fighting, has now been cut.

The Sri Lankan military said intercep­ted radio communications had revealed the mass suicide plan. The military also said that the wife of the Sea Tiger leader, Soosai, who commanded the rebels' naval forces, had suggested under questioning that he and other senior figures remained inside the no-fire zone.

The Sri Lankan army assault went ahead despite a warning from Gordon Brown, that "there will be consequences for its actions". Downing Street said the prime minister had made several phone calls to the Sri Lankan president, Mahinda Rajapaksa, urging the need for an end to violence and aid for civilians.

Humanitarian agencies, meanwhile, were in despair as sporadic reports emerged of thousands of civilians being killed inside the no-fire zone.

James Elder, the Unicef spokesman in Sri Lanka, said those who remained in the no-fire zone were at the mercy of "indiscriminate firing" from all sides. "It is a bloodbath. It is a catastrophic situation," he said. "We are seeing a complete disregard for civilian life. It is hard to think of a worse place on earth to be right now than on that stretch of beach."

About 20,000 people are believed to have escaped from the no-fire zone between Thursday and afternoon, but Elder said many of those who had managed to get out were in a terrible condition. "When you look at the state of the first people to leave three weeks ago, there were malnourished children and women, and people with gunshot wounds and shrapnel injuries, and these people now have been there for another three weeks with next to nothing to eat in terrible conditions. It is going to be a nightmare," he said.

Gordon Weiss, the UN spokesman, said reliable reports from inside the war zone had dried up after the "courageous" doctors who had been working out of the last makeshift hospital at Mullaivaikal East primary school were forced to abandon the building in the face of heavy fighting yesterday. "We are most concerned about the fate of the 30,000 to 80,000 people who are left inside the combat zone," he said. "This is precisely the situation we feared all along - that they would be left inside at the penultimate moments of the battle."

Despite the mounting death toll, neither side in the conflict showed any willingness to lay down arms to allow the trapped civilians to escape. The Tigers said in a statement that they were "extremely mindful of the civilian hardships" and were "prepared to take all necessary measures that would immediately stop the current carnage". They said that "an onslaught by the government will only result in thousands more dying and will not pave a way for a dignified and respectful outcome".

The Sri Lankan military said it would press on with what it described as a humanitarian operation. Brigadier Udaya Nanayakkara, the military spokesman, said: "Operations are continuing to rescue the civilians still being held hostage by the terrorists."

Today the Sri Lankan army completed a pincer movement to surround the Tigers, seizing control of the coastline and cutting off the rebel group's escape route to the sea. The whereabouts of the group's leader, Velupillai Prabhakaran, and other senior commanders are unknown.

The UN secretary general, Ban Ki-moon, has sent his chief of staff, Vijay Nambiar, to Sri Lanka for a second time to try to bring the conflict to a peaceful conclusion. Nambiar was due to arrive last night to hold meetings with senior government officials. The government has brushed off repeated calls from foreign diplomats for a humanitarian truce, saying this would only give the rebels time to regroup.

Attempts by the International Committee of the Red Cross to evacuate thousands of wounded civilians failed last week, with the organisation saying the scale of the fighting made it impossible to get casualties out.