One of these eyewitnesses, an army officer, accuses Defence Secretary Gotabaya Rajapaksa - the president's brother - of ordering Brigadier Shavendra Silva to execute Tamil rebel leaders, whose safe surrender had been guaranteed by the president.
The other new witness, who was also operating with Brigadier Shavendra Silva's 58 Division on the front line during the final assault, claims the Brigadier was ordered by the defence secretary "to finish the job by whatever means necessary."
'Licence to kill'

He said this was interpreted by the soldiers as a licence to kill. He described how he had watched as Sri Lankan forces shot dead unarmed Tamil women and children. It is the first time this allegation has been made.

The war was won by Sri Lankan government soldiers two years ago. The rebel leadership was virtually wiped out.

As the army closed in, around 130,000 Tamil civilians had been trapped on an ever-shrinking shard of land on the north-east of the island alongside the besieged rebel fighters, who are accused of using them as human shields.

The Sri Lankan president said no civilians were killed by the army during the final assault. But the United Nations now believes that up 40,000 civilians were killed during the last few weeks of the conflict. Others estimate the toll to be higher.

Most are thought to have died as a result of gun and mortar fire allegedly directed on them deliberately by government forces.

After repeated requests for an interview with Shavendra Silva - now retired and promoted to the rank of major-general - Channel 4 News went to confront him with these allegations in New York, where he serves as Sri Lanka's deputy ambassador to the UN.