COLOMBO (AFP) - The Sri Lankan government vowed Friday to capture all Tamil Tiger-held territory within 48 hours, despite international calls for a truce and accounts of a "humanitarian catastrophe."

The signal that the final push against the beleaguered separatist guerrillas was in full swing came as United Nations Secretary General Ban Ki-moon's chief of staff was rushing to the island in a fresh effort to stop the carnage.

The International Committee of the Red Cross (ICRC), the only neutral organisation working in the conflict area, said its staff were "witnessing an unimaginable humanitarian catastrophe."

Former colonial power Britain said it wanted an investigation into alleged war crimes, while the United States announced it was blocking a two billion dollar International Monetary Fund bailout package for Sri Lanka.

Tens of thousands of Tamil civilians are believed to be trapped inside a tiny patch of jungle on the northeast coast still held by the rebels. Hundreds have been reported killed in indiscriminate shelling over the past week.

But Sri Lankan government spokesman Anusha Palpita said the war against the remaining rebels would be over by Sunday morning.

"The president (Mahinda Rajapakse) assured that within the next 48 hours the thousands of Tamil civilians will be freed from the clutches of the Tamil Tigers," Palpita said.

"All territory will be freed from Tiger control."

Military officials said the Liberation Tigers of Tamil Eelam (LTTE) were poised to lose their last patch of the island.

"We are closing in from all directions," military spokesman Brigadier Udaya Nanayakkara said.

State media showed footage of fires sweeping through the tropical beach and lagoon battle zone, and said the guerrillas appeared to be retreating and destroying ammunition stock piles.

Sri Lanka's ITN channel said some civilians were managing to escape. A fleeing Tamil woman told the channel that "there are people dead everywhere, on the streets and everywhere."

The government maintains that the Tigers are using civilians as human shields and they need to be rescued. Any civilian deaths inside Tiger territory have been blamed on the rebels.

The UN's human rights chief Navi Pillay, however, has said both sides may be guilty of war crimes.

The ICRC, the only aid organisation that the government allows to work in the conflict zone, issued its darkest statement to date on the current round of fighting.

"Our staff are witnessing an unimaginable humanitarian catastrophe," ICRC director of operations Pierre Krahenbuhl said in Geneva.

He said a boat chartered by the Red Cross to evacuate wounded civilians had not moved for three days because of fierce fighting, while emergency food aid was also stuck off the coast.

The UN has said as many as 50,000 people may be trapped - huddled under plastic sheeting, in shallow bunkers and with little or no food, water or medical facilities.

A top UN envoy, the secretary general's chief of staff Vijay Nambiar, was meanwhile due on the island on Saturday "to help resolve the humanitarian situation," officials in New York said.

Prior peace missions by top diplomats have ended in failure, and on Thursday the Sri Lankan government repeated it would not cave in to pressure.

US Secretary of State Hillary Clinton said this meant it was "not an appropriate time" to consider a massive IMF bailout loan for the island.

The United States is the main shareholder in the IMF and its approval is key to the release of the loan.

In Britain, junior foreign minister Bill Rammell called for a war crimes probe - something already demanded by leading human rights groups.

"The UN's estimate, if it is accurate, of over 6,500 civilian deaths since January is truly shocking and appalling," he said.

"We would support an early investigation into all incidents that may have resulted in civilian casualties... to determine whether war crimes have been committed."

There has not been any word on the LTTE's leadership, in particular rebel chief Velupillai Prabhakaran, who is 54. There has been widespread speculation that he may have already fled by boat.