Secretary of State John Kerry warned Tuesday that Washington is considering a "Plan B" to deal with Syria if Damascus and Moscow are not serious about negotiating a "political transition".

Secretary of State John Kerry warned Tuesday that Washington is considering a "Plan B" to deal with Syria if Damascus and Moscow are not serious about negotiating a "political transition".

Briefing US lawmakers, Kerry said he had told Russia's President Vladimir Putin the United States would not wait more than a few months to see whether President Bashar al-Assad is "serious" about talks.

But he would not be drawn on details of any "Plan B" that he would advise President Barack Obama to adopt if efforts to mediate a political deal to end the Syrian crisis war fail.