Sri Lanka's separatist Tamil Tiger rebels yesterday declared a unilateral ceasefire to end what they said was "an unprecedented humanitarian crisis" but fighting continued as the move was dismissed by the government.

The group made the plea a day after John Holmes, the United Nations under secretary general for humanitarian affairs, arrived in Colombo to push for more protection for tens of thousands of civilians trapped in the remaining pocket of rebel territory.

The Liberation Tigers of Tamil Eelam, who once controlled most of Sri Lanka's north and east are now confined to a 6km coastal strip, The UN estimates the remaining LTTE zone contains about 50,000 civilians.

"In the face of an unprecedented humanitarian crisis and in response to the calls made by the UN, the EU, the governments of the US, India and others . . . all of the LTTE's offensive military operations will cease with immediate effect," said a statement from the group's political headquarters carried on pro-rebel website, Tamilnet.

The government of Sri Lanka's nationalist president Mahinda Rajapaksa said the offer was meaningless and troops continued closing in on the remaining LTTE cadres and their elusive leader, Velupillai Prabhakaran.

The UN says 6,500 refugees have been killed in the latest phase of the fighting, in which the rebels have been caught in a rapidly shrinking pocket of jungle in the island's north-east.

Growing international concern about the fate of the refugees prompted Mr Holmes to travel to the island for a three-day visit at the weekend.

Both sides allege that the other has been using the civilians as human shields. More than 108,000 have escaped the area this month amid allegations the LTTE shot at them as they tried to flee to the government's lines.

Brigadier Udaya Nanayakkara, Sri Lankan army spokesman, told the Financial Times the fighting had moved into the vast refugee encampments in the remaining LTTE-held area, which the military earlier had set aside as a "no fire" zone.

"They [the LTTE] have not stopped shooting," Brigadier Nanayakkara said last night when asked about the ceasefire. "They are using the refugee tents. Disguised as refugees, they are waiting for the troops to come then firing at them."

Most analysts view the LTTE's ceasefire statement as a tactical ploy to buy time as its leaders watch their dream of an independent homeland for ethnic Tamils in Sri Lanka's north and east rapidly disappearing.

"Death and destruction continues unabated. This has been made worse by the deliberate withholding of food and medicine over many months. Deaths due to starvation are imminent," the LTTE statement said.

Sri Lanka's defence chief dismissed the Tamil Tigers's declaration of a ceasefire as a "joke", and said the guerrillas must surrender or be destroyed.

"There is no need of a ceasefire. They must surrender. That is it," Gotabaya Rajapaksa, defence secretary, told Reuters.