Peace talks on Syria due to start this week in Switzerland are in disarray partly due to the ambiguity of a 2012 Geneva accord which calls for an interim government but is unclear on the current president's future.

World powers had agreed in a meeting convened by former UN secretary general Kofi Annan at the end of June 2012 on a power transition plan to end the bloody crisis in Syria.

But the silence on President Bashar al-Assad's future has proved to be a major sticking point, compromising the so-called Geneva II talks due to start on Wednesday.

Several Western countries, including the United States, France and Britain, interpret the Geneva I accord as evoking a post-Assad scenario.

But Assad's ally Russia, which along with the United States has been a main backer of the Geneva II talks, as well as China -- another permanent member of the UN Security Council -- say it is up to the Syrians to determine their fate.

The Geneva I accord stipulates that the transition authorities could include members of the present government and the opposition and other groups and should be formed on the basis of mutual consent.

It could proceed to make constitutional changes, which would be put to a popular vote before free, fair and transparent elections are held.

The plan also called for an end to the bloodshed, for sending humanitarian aid to the worst-affected regions and releasing prisoners.

Victims of the conflict would be compensated by the courts and all government institutions would respect human rights.

But the communique has fuelled divisions between the so-called Contact Group which was at the heart of the Geneva I initiative.

The Group was comprised of the UN Security Council's five permanent members -- Britain, China, France, Russia and the United States -- plus states representing the Arab League, Turkey, and leaders of the European Union, the Arab League and the United Nations.

This week's talks to be held in Montreux in Switzerland before moving to Geneva have hit stumbling blocks over UN Secretary General Ban Ki-moon's surprise invitation to Assad's ally Iran to attend the talks, and the Syrian leader's refusal to hand power to the opposition.