UN monitors will immediately begin a mission in Syria to investigate the use of chemical weapons, UN Secretary General Ban Ki-moon said.

"The mission is expected to begin conducting on-site fact finding activities today (26th of August) in just matter of hours. We have all seen the horrifying images on our television screens and through social media. Clearly this was a major and terrible incident. The whole world should be concerned about any threat or use of chemical weapons. And that is why the world is watching Syria," the Secretary-General said.

Although the Syrian government has denied responsibility for the deaths, the video prompted international outrage.

It has claimed chemical agents were found in tunnels used by the opposition forces.