Footage shot by an independent journalist for Britain's ITV News appears to show victims of an alleged chemical attack that activists said killed hundreds of people.

A small crater and exploded metal was only feet away from where dead bodies were lying.

An unnamed survivor of the attack, shown in the video, said he had gone to investigate a small explosion that he heard, but smelt a strange odour.

"The next morning we discovered all families dead in their homes. [I] just opened the door [to] see the whole family killed in place with foam in their mouths and eyes together with blood in their ears," he said.

Another witness said that the explosions from the 12 missiles that fell on the area were not loud, so residents thought the attack was further away.

Opposition activists have said that a number of people who gave medical help to victims died due to exposure to chemicals.

Chemical weapons experts have said that this could point to the possible use of a nerve agent.

"By looking at the videos you can tell that some of the victims are suffering from asphyxiation, they don't have any external wounds, so it's consistent with the use of a chemical agent," said Dina Esfandiary of the International Institute for Strategic Studies.

She added that it was difficult to say what actually happened without samples.

The UN said in a a statement on Friday that Angela Kane, the High Representative for Disarmament Affairs, was due to arrive in Damascus on Saturday to push for access to the site of the attack for UN inspectors, who are already in Syria to investigate previous chemical attack claims.