New satellite images appear to show evidence of recent heavy shelling in a government-designated civilian safe zone in Sri Lanka, a rights group says.
The images show crater marks and considerable population displacement between 6 and 10 May.
Human Rights Watch says the images and witness testimony they have gathered contradict troops' claims that they are not using heavy weaponry.
The army disputed the analysis, saying Tamil Tiger rebels caused the damage.
The claims are impossible to verify as reporters are banned from the area.
Human Rights Watch blamed both sides for the continuing bloodshed.
"Neither the Sri Lankan army nor the Tamil Tigers appear to have any reluctance in using civilians as cannon fodder," said Brad Adams, its Asia director.
But the organisation said witness testimony indicated that recent heavy fire had come from the direction of government-controlled areas.
Analysis of satellite imagery, commissioned by Human Rights Watch and Amnesty International, indicated significant changes in population movement on the ground in recent days.
Satellite images taken on 10 May after a night of reported heavy fire were compared with images of the same area taken four days earlier.
"By comparing before-and-after satellite images, we were able to see a significant movement of the region's human population, suggesting widespread displacement," said Lars Bromley of the American Association of the Advancement of Science.
"We also saw destroyed structures and circular, crater-like features consistent with widespread shelling."
One area which had been packed with tents and other structures in the earlier photo was virtually empty in the image taken on 10 May.
The report said that it was "certainly unlikely that the IDPs would have moved en masse, and so completely without a compelling reason".
It would be difficult to say exactly what caused the population displacement or who might be responsible for the damage from the images, according to the report.
Sri Lanka's military spokesman, Brigadier Udaya Nanayakkara, blamed the rebels for the crater damage.
"The LTTE have enough explosives. They have booby-trapped buildings as well... this is what caused the craters," he said.
He attributed the population displacement to "the rebels forcing the civilian population to move along" as the battle front advanced.
On Tuesday the rebels accused the army of shelling a hospital, killing 49 people.
The US and UK have urged Sri Lanka's government and Tamil Tiger rebels to stop fighting "immediately" and allow an evacuation of trapped civilians.
The UN's humanitarian co-ordinator John Holmes said intransigence by both the Sri Lankan government and the rebels had created an "absolutely awful" situation.
The UN estimates that about 50,000 civilians are trapped by the conflict, in a three-sq-km strip of land as the rebels and the government battle in the final stages of the conflict.