The Sri Lankan military on Thursday accused Tamil Tigers of using phosphorus bombs as part of their last-ditch attempt to face off against a massive onslaught by government troops.
A military statement said the once-powerful rebel force, now surrounded in a four square kilometre (1.5 square mile) patch of jungle on the northeast coast, had deployed the bombs as landmines across their defence lines.
Phosphorus bombs are incendiary weapons which are banned from use in civilian areas under an international convention.
The allegation came after doctors treating civilians who have fled the fighting reported that some burn injuries suggested that the weapons were being used -- although medical staff have not blamed either the rebels or the government.
The island's defence ministry pointed the finger at the rebels.
"Terrorists are burying improvised bombs that would ultimately result in a humanitarian catastrophe, with the bomb explosions leading to phosphorous burns on both civilians and advancing troops," a statement said.
"The use of such cowardly tactics is indeed worsening the unnecessary suffering of the hostages held at gunpoint and the indiscriminate use of such incendiary agents by the LTTE is reprehensible."
Tamil Tiger rebels have also accused government forces of using chemical weapons, while both have denied they posses such devices.
The rebels have drawn widespread international condemnation for using tens of thousands of Tamil civilians as human shields and shooting those who try to escape.
But the government has also been on the defensive over accusations that its troops have carried out indiscriminate shelling of the rebel-held zone, resulting in soaring civilian casualties.