US Secretary of State John Kerry has described the situation in Syria as "our Munich moment", comparing possible military intervention to the 1938 Munich Agreement, which ceded control of part of Czechoslovakia to Nazi Germany.

Speaking in Paris, he said that he and President Obama are "exceedingly grateful to have France by our side" in backing a political solution and "limited" military action over Syria.

"This is not the time to be silent spectators to slaughter", he said. "This is not the time to send a message where doing nothing is far more risky than responding."