The United Nations tells Channel 4 News new footage of the alleged massacre of Tamil prisoners in Sri Lanka "deserves more investigation", as Sri Lankan President Mahinda Rajapakse visits London.

Sri Lankan President Mahinda Rajapakse is in London as new war crimes allegations surface

Last night, Channel 4 News broadcast an extended Sri Lanka execution video, apparently showing government troops executing Tamils in the last few weeks of the Sri Lankan civil war.

The video is a longer version of one which already sparked a United Nations (UN) investigation 16 months ago when it first came to light.

The UN Special Rapporteur on extrajudicial, summary or arbitrary executions, Professor Christof Heyns, told Channel 4 News futher investigations were needed.

"It is shocking indeed, and clearly deserves more investigation," he said.

Channel 4 News has sent the new video to the UN panel investigating allegations of war cimes in Sri Lanka.

The country's 26-year civil war ended in May 2009 when the Sri Lanka army defeated the LTTE - Tamil Tigers - in an area known as the "no-fire zone". Estimates of how many civilians died in just the last five months of the conflict, when violence escalated hugely, range from 7,000 to 30,000.

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The video appears to show the same incident as one aired 16 months ago by Channel 4 News, which the Sri Lankan government says is a fake. It shows the execution of men in a muddy field, allegedly defeated Tamil prisoners. A UN investigation later found that the video "appeared authentic".

The new video seems to show the same incident but rather than stopping after the execution of a second bound man, it continues and the camera pans left to reveal the naked and dead bodies of at least seven women, with accompanying dialogue from onlookers who make lewd and callous comments which seems to strongly suggest that sexual assaults have taken place before the death of the women.

Sri Lanka execution video: new war crimes claims

Channel 4 News Foreign Affairs Correspondent Jonathan Miller said: "The rest of the video is too gruesome to broadcast: women bound, shot dead, undressed; callous comments from onlookers laden with sexual innuendo. Since Channel 4 News received and broadcast the executions video over a year ago we have received hundreds of photographs and many more shocking videos depicting summary executions and rape.

"We've now sent this five minute 30 second video to the UN panel convened to determine whether or not there should be an independent international war crimes inquiry."

President's visit

The video has emerged as the Sri Lankan President Mahinda Rajapakse visits London on a private trip.

He was due to speak at the Oxford Union but his appearance there has been cancelled by the Union over "security concerns".

His spokesman told Channel 4 News that the President met today with Defence Secretary Liam Fox, in a private capacity.

Video

In a statement, the Sri Lanka High Commission denied the authenticity of the new video.

"The High Commission of Sri Lanka categorically denies that the Channel 4 News TV video is authentic. Last year when Channel 4 News telecast a similar video the Government of Sri Lanka clearly established, by reference to technical considerations, that it was not genuine but fake.

"The present video is nothing more than an elongated version of the same video."

Read the Sri Lanka High Commission statement in full here.