At least 25,000 civilians have fled a Tamil Tiger-held area in northern Sri Lanka, the military says.
People escaped after troops broke through a fortification which had been blocking their advance into the Tigers’ last stronghold, the army said.
Aerial video showed thousands of people filing out of the combat zone. Tens of thousands remain in the area, which has seen heavy fighting for months.
The government says the rebels must now surrender or face a final assault.
There was no immediate response from the rebels, who have rejected previous calls to surrender. The pro-rebel TamilNet website said several hundred civilians were feared killed and injured after troops advanced into the zone.
Each side accuses the other of killing civilians in the long running civil conflict.
Foreign reporters are not allowed into the combat zone, making it impossible to independently verify the claims.
The Red Cross (ICRC) says government doctors working in the area are "worn out" and do not have enough supplies.
The BBC’s Charles Haviland in Colombo has seen the video, shot by an unmanned military vehicle, of civilians fleeing the combat zone. The film was apparently taken early on Monday morning.
Confirming estimates of at least 25,000 people fleeing, our correspondent described as "haunting" the sight of thousands of people filing as quickly as they could away from the ocean side of the strip of land where fighting has been going on.
A far greater number were visible milling around outside the zone waiting to be taken to government-run camps, our correspondent says.
Some estimates say as many as 35,000 people fled the fighting on Monday.
President Mahinda Rajapakse said: "The footage clearly shows that the people are defying the LTTE [Liberation Tigers of Tamil Eelam] and escaping. They are running to safety.
"The only thing [LTTE leader Velupillai] Prabhakaran can now do is to surrender."
‘Worn-out’ doctors
Earlier in the day, military spokesman Brigadier Udaya Nanayakkara said the army had managed to smash through an earth fortification which had been blocking its advance into the tiny coastal strip held by the rebels.
The military spokesman said those who had just escaped would go, like others, to special camps around the town of Vavuniya about 100km (60 miles) from the fighting.
Our correspondent says life for up to 100,000 Tamil civilians in the coastal strip, less than 20 sq km in size, is a nightmare.
There has been shelling there for months, while the UN says the Tigers are preventing people from escaping, despite rebel denials.
The government is not giving the ICRC access to the landward side of the zone.
So it can evacuate people only by sea, with two or three ships per week each carrying 400 or 500 of the sickest, oldest and most badly wounded people.
"The situation there is very difficult. The makeshift health facilities that are still existing are not immune of the effects of the hostilities," ICRC country chief Paul Castella told the BBC.
"The ministry of health staff still working there are really worn out – these people are working for months now without any break, they work day and night. And medical supplies are lacking," he added.
Mr Castella said people were densely packed in the area of hostilities, totally reliant on external aid and lacking sanitation.
The Red Cross has evacuated more than 10,000 civilians since early February, but Mr Castella could not confirm or deny figures that others are citing for the number of people recently killed, or managing to escape by land.
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