The Tamil Tigers declared a unilateral ceasefire on Sunday, but Sri Lanka dismissed it as a 'joke' and said only a surrender would stop troops from finishing the last battle in Asia's longest modern war.

The Tigers' truce declaration came as the UN's top humanitarian chief was in the Indian Ocean island to press for the protection of tens of thousands of people trapped in the apparent final conventional battle of a war that started in 1983.

And Sri Lanka's ruling party won a resounding victory in a provincial poll, seen as the latest referendum on president Mahinda Rajapaksa's war effort and another step to shoring up his power before possibly calling an early national election.

The Tigers have offered a ceasefire repeatedly as the military juggernaut has pushed them to the brink of defeat, but have refused international calls to free stranded civilians, whom witnesses say are kept from leaving by deadly force.

'In the face of an unprecedented humanitarian crisis and in response to the calls made by the UN, EU, the governments of India and others, the Liberation Tigers of Tamil Eelam (LTTE) has announced a unilateral ceasefire,' an LTTE statement said.

Sri Lanka's defence secretary, the top civilian official in charge of the military and the president's brother, laughed at the truce declaration.

'That is a joke. They were not fighting with us, they were running from us. There is no need of a ceasefire. They must surrender. That is it,' Defence Secretary Gotabaya Rajapaksa told Reuters in a telephone interview.

The war success has driven Mahinda Rajapaksa's popularity high, and helped him sideline the main opposition United National Party (UNP). Election results from the Western province showed his party got more than double the seats of the UNP.

But after the end of the conventional war, Sri Lanka will face challenges healing divisions between the Tamil minority and Sinhalese majority and boosting an ailing economy. It is seeking a $1.9 billion IMF loan to ease a balance of payments crisis.

UN chief visit

Colombo has long said the LTTE must either surrender or face annihilation, and says the rebels' previous use of ceasefires to re-arm proves their bad faith. The United Nations, the United States, European Union and others are urging a new truce.

'We are of the view that only such a ceasefire can end the humanitarian crisis and help avert the long-term impact of this crisis on the region and on the peoples of the island,' the LTTE statement said.

It made no mention of surrender, nor of releasing the people still inside the battle zone, whom the rebels say are being killed in Sri Lankan military assaults.

The military denies targeting civilians, and says the LTTE is hiding behind them as a human shield.

Since LTTE founder-leader Vellupillai Prabhakaran commands followers to wear vials of cyanide to be taken in case of capture, surrender is viewed as highly unlikely.

He is believed to be in the 10 sq km the LTTE still controls, all that remains of the separate nation they have been fighting since the early 1970s to create for Sri Lanka's Tamil minority.

Diplomatic pressure on the government has risen sharply to protect the welfare of civilians held by the LTTE in fighting, even after 109,000 fled en masse when troops blasted an earthen barricade on Monday blocking entry and exit to the rebel area.

Internal United Nations tallies say nearly 6,500 have been killed in fighting since the end of January, and diplomats say they are increasing the pressure on Colombo since the LTTE has refused all entreaties so far and has little left to lose.

The United Nations, the United States and others have accused the military of shelling civilian areas and the LTTE of forcibly recruiting people to fight while shooting those who t