The 24-hour ultimatum came to an end at noon on April 21 without a response from the rebels, who are cornered on silver of the coast and facing total defeat after a 25-year fight for an independent, ethnic Tamil homeland.

Human rights groups say tens of thousands of civilians remain trapped in the rebel-held enclave and fear the civilian death toll could rise if the military launches a final assault.

On Monday, Sri Lankan soldiers broke through a barrier that the Tamil Tiger rebels had erected to defend their ever-shrinking slice of territory.

A total of 39,081 civilians fled the war zone on Monday, the largest exodus in a single day, said military spokesman Brig Udaya Nanayakkara. At least 10,000 people crossed over on April 21, and numbers are expected to rise, he said.