The international envoy to Syria Kofi Annan is to meet with US Secretary of State Hillary Clinton to discuss the situation in Syria.

PJ Crowley, a former assistant secretary of state under President Obama's administration, says a shift in Russia's approach to the conflict is "critical".

A Yemen-style solution where Russia encourages President Assad to step down would be "conceptually a good way to resolve" the conflict although there are "very significant obstacles".

He told the Today programme that the US has been "somewhat fooled" by the speed of the transitions in other middle-eastern countries affected by the Arab Spring.

The Syrian conflict is "much more complex and dangerous", he warns and it "may have to unfold tragically for some period of time", as in Bosnia, before a solution is found.

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