Colombo - The United Nations and Red Cross on Thursday helped evacuate about 350 critically ill or injured people from areas in northern Sri Lanka where fighting between government troops and Tamil rebels is raging, a medical doctor based in the north said.

The patients and the injured were trapped in the Pudukudirippu hospital, 385 kilometres north-east of Colombo.

Earlier, the government had claimed that the Tamil rebels had prevented the International Committee of the Red Cross (ICRC) from moving out the civilians because the separatist guerillas did not want civilians leaving the areas still under their control.

But officials in the Liberation Tigers of Tamil Eelam (LTTE) rebel group charged that that ambulances were not allowed into the area by government security forces.

"We have been told to make arrangements to receive the persons being transferred by the UN and the ICRC," said a doctor at the Vavuniya hospital, located about 90 kilometres south of where the fighting is taking place.

He said that among those being transferred were children but he did not have specific figures.

Other sources said around 50 children were being transferred for treatment.

The convoy carrying the patients consisted of three ambulances, four lorries, four buses and three jeeps belonging to the United Nations, and it was due at Vavuniya hospital later Thursday.

Earlier, the Red Cross and United Nations expressed concern about the civilians caught in the conflict in the north.

They estimated that around 250,000 civilians were trapped in the rebel-held areas there, but the government said the figure might be around 150,000.

Meanwhile, as the fighting continued, government forces recovered what they described as "four underwater vehicles" that could be used for sea operations in the Tamil rebels' battle against the Sri Lankan navy.

"Three suicide boats and a large haul of LTTE maritime equipment were also found at the location," the Defence Ministry said in a statement.

The largest underwater vehicle found was about 10.6 metres in length and was amour-plated, the ministry said.

The government claims the rebels, who have been fighting for 26 years for autonomy for the minority Tamils in majority-Sinhalese Sri Lanka, are now trapped in a 300-square-kilometre area in the Mullaitivu district of northern Sri Lanka, their last remaining stronghold.

The government has captured vast areas formerly held by the rebels in the northern and eastern parts of the country in their operations launched in August 2006 and have now entered what they describe as the final phase of their offensive.

The insurgency has claimed more than 76,000 lives and displaced tens of thousands of people. (dpa)