Internally displaced Sri Lankan ethnic Tamil civilians line up to receive food at a camp for the displaced in Manic Farm in Vavuniya on Sunday. (Sanath Priyantha/Associated Press)

Sri Lanka's Tamil Tiger rebels declared a unilateral ceasefire on Sunday, but the country's defence minister called the announcement "a joke" and said the rebels must surrender.

The military has the guerillas — and an estimated 50,000 civilians — surrounded in a tiny coastal area of northeastern Sri Lanka.

The Liberation Tigers of Tamil Eelam say they're calling for a ceasefire because an unprecedented humanitarian crisis is unfolding. Aid groups say some of those trapped are dying of starvation.

Defence Secretary Gotabaya Rajapaksa said there is no need for a ceasefire. He said rebels are not fighting the military but running from it.

The Sri Lankan army has said a pause in the fighting will only give rebels time to regroup.

Sri Lanka's government has accused the rebels of turning civilians into hostages within a territory of less than eight square kilometres.

Reports of chaos and suffering in the war zone have increased in recent days as the Sri Lankan military has pushed forward with its offensive to destroy the separatist insurgency.

UN official calls for access to war zone

UN humanitarian chief John Holmes arrived in the capital, Colombo, on Saturday for a three-day visit.

During a meeting with government officials on Sunday, he urged a reopening of the war zone to humanitarian groups. Aid workers have been barred from the region since fighting escalated last year.

On Monday, Holmes plans to travel to the northern region of Vavuniya to inspect displacement camps and hospitals that have been overwhelmed by more than 100,000 civilians who fled the war zone over the past week.

The United Nations says nearly 6,500 civilians have been killed in the fighting over the past three months.

Holmes said Saturday that the trapped civilians were suffering a "very high" casualty rate, and from lack of food, clean water and medical supplies.

The government insists it has sent food and medicines and accuses the rebels of holding the civilians as human shields.

It is not possible to verify the claims because the government has barred independent journalists from the war zone, arguing that it is too dangerous for them to work.

With files from The Associated Press