Sri Lankan government forces killed more than 250 civilians in artillery attacks in the last patch of territory held by the Tamil Tigers, the pro-rebel Tamilnet website reported on Sunday.
The website said 257 bodies had been brought to a makeshift hospital in the area after what it said was shelling by government forces overnight Saturday.
The military swiftly denied the report.
"We have not used heavy weapons in the area where the Tamilnet says civilians had been killed," military spokesman Brigadier Udaya Nanayakkara said.
The Tamil Tiger rebels and the military regularly accuse each other of atrocities in the civil conflict - claims that are impossible to verify.
Foreign reporters are banned from the war zone.