Tamil Tiger rebels had just hours left to surrender by a government deadline on Tuesday, as tens of thousands of civilians fled the sliver of jungle the guerrillas still control in northeast Sri Lanka.
With international concern growing over the fate of civilians caught up in the conflict, the rebels could be facing a last push by troops seeking to end their 37-year armed campaign for a Tamil homeland.
The defence ministry gave Tiger supremo Velupillai Prabhakaran and his fighters until 0630 GMT on Tuesday to surrender, although the government did not specify what action would be taken otherwise.
The warning came as aerial video footage provided by a Sri Lankan spy plane showed more than 35,000 civilians streaming out of the jungle strip where the Liberation Tigers of Tamil Eelam (LTTE) are cornered.
Their presence has been cited by the Sri Lankan military as a main reason preventing a final assault on the remaining Tigers in a government-designated "no-fire zone."
President Mahinda Rajapakse said the rebels were facing "complete defeat" and their reclusive leader had finally run out of time.
"The only thing Prabhakaran can now do is to surrender," he told AFP. "I don't want him to take cyanide and commit suicide. He has to face charges for his actions."
The government has issued similar ultimatums in the past, most recently on April 12 when the guerrillas were told they had 48 hours to surrender.
So far, the only reaction from the Tigers has been to renew their call for an unconditional ceasefire -- something the government has already rejected.
The crisis has sparked protests in Europe and expressions of concern by UN and human rights groups.
In Paris, French police arrested 210 people on Monday when a rally by Tamils turned violent as demonstrators threw bottles at security forces and smashed windscreens.
In London, thousands of Tamils blocked some of the city's busiest streets, demonstrating outside parliament and calling for an immediate ceasefire.
New York-based Human Rights Watch (HRW) warned that the world had only hours to prevent a potential "bloodbath" in Sri Lanka.
Anna Neistat, a senior HRW researcher, said the international community had "to make it crystal clear to both sides of this conflict -- both the LTTE and the government of Sri Lanka -- that they will not get away with it."
But Jaliya Wickramasuriya, the Sri Lankan ambassador to the United States, insisted the government's priority was to rescue civilians.
"We are not going to do any huge operation as such," he told AFP. "We are still concerned about civilians."
UN Secretary General Ban Ki-moon welcomed the escape of the civilians but was "deeply concerned" about those still trapped, his office said.
According to the United Nations, as many as 100,000 civilians were trapped in LTTE territory and living in "dire humanitarian conditions" before Monday's exodus.
The mass civilian escape shown in the aerial footage appeared to have been triggered by the Sri Lankan military overrunning a Tiger defensive embankment.
The defence ministry said the Tigers killed 17 civilians on Monday in at least one suicide bombing against escaping civilians.
Both sides have traded accusations of targeting non-combatants, and the international community has repeatedly urged a permanent ceasefire to prevent further loss of innocent lives.
The LTTE -- once seen as one of the world's most efficient guerrilla units -- used to control a third of Sri Lanka's territory, an overseas fund-raising network and a lucrative shipping business.
In Colombo, President Rajapakse's office compared the exodus of civilians to the fall of the Berlin Wall.
"The exodus to freedom that is taking place in the north of Sri Lanka just now is no less a massive vote for freedom than the fall of the Berlin Wall in November 1989," it said in a statement.