Colombo  - Government forces have stepped up operations to capture the last rebel-held area in northern Sri Lanka after rejecting an unilateral ceasefire offered by the Tamil insurgents, a pro-rebel website said Monday.

Troops have launched operations from the north, west and south on Mullivaikkal area, a narrow strip on the coast area of the Mullativu district, 395 kilometres north-east of the capital Colombo, the Tamilnet website said.

The report said the operation had been launched around 3:45 am Monday (2215 GMT Sunday), and heavy gunfire was reported in the area.

Military officials confirmed they were continuing their operations but declined to comment whether they had stepped up the attack.

The Liberation Tigers of Tamil Eelam (LTTE) rebels have claimed that an estimated 160,000 civilians are pinned down in the coastal area, but the government says there are only 20,000 civilians still trapped. Other sources say there may be around 40,000 people in the war zone.

"If the expected offensive goes forward, there will be more than 10,000 casualties as the area is densely populated and there is no cover from bombs, shells, and bullets," the Tamilnet quoted a press statement issued by the Tamils Rehabilitation Organisation (TRO), a front organization for the LTTE.

Civilians trapped in the area face severe shortages of food and medicine, as the last aid transport, a ship sailing under a Red Cross flag, reached the area on April 2.

Last week some 109,000 civilians fled from the combat zone into into the government-controlled areas raising the total of number of refugees to 170,000 since January.

Tamil rebels announced a unilateral ceasefire on Sunday, but Defence Secretary Gotabaya Rajapaksa rejected the offer saying the rebels must surrender.

The defence secretary, who is the brother of President Mahinda Rajapaksa, vowed to fight until the LTTE is crushed.

Visiting UN humanitarian chief Sir John Holmes also called for a humanitarian pause enabling civilians to leave the combat zone, but his call was so far ignored by the Sri Lankan government.

Holmes is due to meet with Foreign Minister Rohitha Bogollagama Monday to further discuss the trapped civilians' plight as well as measures to assist those who have already fled the area and are living in government-controlled camps.

Meanwhile foreign ministers from Britain, France and Sweden are to visit Sri Lanka on Wednesday.

"Foreign Secretary David Miliband, together with his French counterpart Bernard Kouchner and Swedish counterpart Carl Bildt, will visit (Sri Lanka) on Wednesday," a statement from the British High Commission said.

On Sunday British Prime Minister Gordon Brown telephoned President Rajapaksa to express his concern about the plight of the civilians trapped in the rebel area.

The military says they are on the final phase of crushing the LTTE who have been fighting in the north and eastern parts of Sri Lanka for an independent homeland for minority Tamils from the Sinhalese majority for the last 25 years. (dpa)