"I don't believe that we should shy from this moment: the risk of not acting is greater than the risk of acting," US Secretary of State John Kerry said in London.
Kerry said it is Assad who will not negotiate as long as he is not prevented from using chemical weapons.
"If one party believes he can rub out countless numbers of his own citizens with impunity...He will never come to a negotiating table," Kerry said.
The US has alleged that the nerve agent sarin was used by the Assad regime on August 21 and that at least 1,429 people were killed, including over 400 children, a charge denied by the Syrian government.
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Russia, which has vehemently opposed unilateral action in Syria, today warned military strikes against Syria risked causing an "outburst of terrorism" in the region.
"The possibility for a political solution remains," Lavrov said. He said that his Syrian counterpart has assured him at the talks in Moscow that Damascus was still "ready for peace talks".
Meanwhile, Assad has warned that if President Obama decides to launch military strikes on Syria, the US and its allies should "expect every action" in retaliation.
"You should expect everything. Not necessarily from the government," Assad told CBS News yesterday in his first television interview since Obama sought congressional approval for military action in Syria.
"You have different parties, you have different factions, you have different ideology. You have everything in this region now," Assad said.