Doctors Without Borders said hospitals were running low on drugs and were short staffed, while ambulances were having trouble moving across the front lines to evacuate the wounded.

Human Rights Watch accused the rebels of keeping civilians in the area, and accused the Sri Lankan government of detaining those who managed to flee in ``militarized detention camps.''

``Civilians are scrambling for shelter in an area that is under heavy artillery fire, including many children, wounded, and elderly who need urgent assistance,'' said Brad Adams, the New York-based group's Asia director.

The Tamil Tigers have fought since 1983 to create a separate state for minority Tamils, who have suffered decades of marginalization at the hands of governments controlled by the Sinhalese majority. More than 70,000 people have been killed in the civil war.

Government forces have pushed the rebels into a shrinking area of about 115 square miles (300 square kilometers) of jungle and villages. The Red Cross estimates 250,000 civilians are trapped there as well. Published: Thu Jan 29 07:15:37 EST 2009

It was detected in a boat factory as troops advanced into the area of Udayarkattukulam 370 kilometres north east of Colombo.

The operation also unearthed three more smaller underwater vehicles still under construction, three boats which could be used for a suicide attack, and a Dvora type fast-attack craft.

The largest underwater vehicle was about 35 feet long and armour-plated, officials said.

``This was the most startling recovery made by troops so far during the ongoing counter-terrorist operations against the LTTE'', the defence ministry said.

``With this discovery the LTTE will go down in the history as the first terrorist organization to develop underwater weapons'', the ministry added.

It posted pictures of the vessel on its official website (defence.lk). Earlier, advancing troops found a bomb- and mine-manufacturing facility. dpa ad sc Published: Thu Jan 29 11:26:52 EST 2009

But the fighting has left hundreds of thousands of noncombatants trapped in the conflict zone north of Mullaitivu, the last rebel town to fall to government forces on Sunday. The de facto rebel capital, Kilinochchi, fell on Jan. 2.

The Red Cross estimates 250,000 civilians are trapped in the territory still under rebel control. Human rights groups have accused the rebels of keeping them as human shields, a charge the Tigers deny.

``The overall humanitarian situation remains precarious for thousands,'' the Red Cross said in a statement. ``Stocks have been depleted and sustainable ways of producing food locally have become almost nonexistent.''

In a joint statement, several Sri Lankan civic groups said they were ``gravely concerned'' about mounting casualties as ``fighting intensifies over the coming days, and the civilians get trapped into smaller spaces.''

They called on the government to allow an international team access to the conflict zone to assess the humanitarian needs, and ensure urgent delivery of food and medicine.

They also urged the Tamil Tigers to allow civilians to leave the area.

On Wednesday, ground troops overran a key crossroads after heavy fighting, military spokesman Brig. Udaya Nanayakkara said.

The junction, just outside the village of Vishwamadu, lies along one of the few major roads remaining in rebel-held territory.

The army also captured several rebel camps including a bunker complex with a luxury room and conference hall in fighting throughout the day Wednesday, the military said.

Troops captured an armored personnel carrier in one camp and in another found an armored submarine, three partially built mini-submarines and an attack boat, the military said.

The rebels could not be contacted for comment because most communication to the north has been cut. Independent accounts of the fighting are not available because most journalists are barred from the war zone.

On Thursday, the United Nations evacuated hundreds of wounded civilians from the conflict zone.

U.N. spokesman Gordon Weiss said a convoy transporting injured people was held up by rebels Wednesday before being allowed to cross into government territory on Thursday morning.

It included ``50 critically injured children on board and some hundreds of (other) critically injured,'' he said.

Dr. Thurairajah Varatharajah, the top health official in the war zone, said at least 1,140 civilians were wounded in the past week and estimated at least 250 were killed in the fighting.

The United Nations said some staff members and their relatives were huddled in a bunker over the weekend in a government-designated ``safe zone'' when artillery shells landed nearby and killed nine people taking shelter 30 feet (10 meters) away.

An internal U.N. memo said the shells were apparently fired by government forces.

Military spokesman Brig. Udaya Nanayakkara has said no civilians have been killed, but that some people who were forced by the rebels to build fortifications might have been wounded in crossfire.

Amnesty International said both sides appeared to be violating the laws of war by endangering civilians.

``The immediate priority is medical attention for the seriously wounded. The Tamil Tigers must let injured civilians go,'' said Yolanda Foster, Amnesty International's Sri Lanka - researcher. ``Preventing civilians from accessing medical care constitutes a war crime.''

The medical aid group Doctors Without Borders said hospitals were running low on drugs, while ambulances were having trouble moving across the front lines to evacuate the wounded.

The civil war grew out of complaints by ethnic minority Tamils, who have suffered decades of marginalization at the hands of governments controlled by the Sinhalese majority. Several attempts by successive governments to reach a political settlement with the Tigers failed. Published: Thu Jan 29 11:27:10 EST 2009