Sri Lanka has given Tamil Tiger rebels 24 hours to surrender after thousands of civilians fled the war zone when troops breached an earthen fortification blocking their exit route.

President Mahinda Rajapaksa said there were about 35,000 people waiting to leave the military-controlled area.

Aerial footage showed thousands of people thronging around temporary reception centres set up by the army less than a kilometre outside the no-fire zone.

"With the influx of people, we have given a final warning to rebel leader Velupillai Prabhakaran and his terrorist group to surrender to the government forces within 24-hours from 12 noon," said defence spokesman Keheliya Rambukwella.

"Thereafter will be a military course of action. That is the best option."

The military said three suicide bombers had attacked thousands of Tamil civilians.

"At least 17 civilians, including women and children, have been killed and 200 people injured from the cowardly suicide attacks," said Military spokesman Brigadier Udaya Nanayakkara.

The wall of earth had blocked the widest land link to the coastal strip where Sri Lanka's military has surrounded the Tigers with the goal of crushing them and ending a civil war that has raged since 1983 and is now Asia's longest-running.

The no-fire zone is a 6.5 square-mile area of coconut groves, where the Liberation Tigers of Tamil Eelam (LTTE) separatists are fighting their last stand from among tens of thousands of civilians they have held there by force.

The military said the exodus of civilians was the largest in a single day.

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