Key Syrian opposition leaders say they will not join peace talks due to begin in Geneva later, aimed at ending the country's civil war.

The United Nations has already warned that the negotiations must not fail.

However, the absence of the opposition leaders threatens the biggest diplomatic push yet to resolve the conflict, which has resulted in the deaths of more than 260,000 people.

The intra-Syrian talks are set to focus on four different areas - political process (governance, process for drafting a new constitution and elections), ceasefire, nationwide humanitarian access and fight against terrorism.

The UN's Special Envoy for Syria, Staffan de Mistura, has recorded a video message for the people of Syria ahead of today's talks.

Mr de Mistura says, "my message today is meant to reach every single man, woman and child of Syria - inside Syria, and outside in the refugee camps or wherever you are".

Saying that the latest round of talks 'cannot fail', he stresses that "five years of this conflict have been too much. The horror is in front of everyone's eyes.

"We are going to not disappoint you from the UN's point of view - we will never abandon the Syrian people," he added.