The US has said it will take action against the Syrian government for alleged use of chemical weapons even without the backing of allies or the United Nations.
Britain added a hurdle to deliberations about a military strike on Wednesday when it went to the UN Security Council with a draft resolution that would authorise the use of military force against Syria.
As expected, the five permanent members of the security council failed to reach an agreement as Russia reiterated its objections to international intervention in Syria.
US state department deputy spokeswoman Marie Harf said the US cannot be held up in responding because of Russia's intransigence at the United Nations. Russia said the use of force without a sanction of the UN Security Council is a "crude violation" of international law.
Earlier, UN chemical weapons experts have visited a Damascus suburb for a new tour of areas struck by poison gas, as Western powers began laying the groundwork for a possible punitive strike.
Meanwhile, UN leader Ban Ki-moon pleaded for more time to allow the team to establish the facts and to give diplomacy another chance to end the crisis. He said the Security Council, whose permanent members are divided over Syria, must not go "missing in action".
Marking the centenary of a venue for peaceful conflict resolution at The Hague, Ban said: "Here in the Peace Palace, let us say: Give peace a chance. Give diplomacy a chance. Stop fighting and start talking."
Two of Syria's staunchest backers, Iran and Russia, warned of dire consequences if the US and its allies attack in Syria. Such strikes "will lead to the long-term destabilization of the situation in the country and the region," said Russian Foreign Minister Sergey Lavrov.
Iranian Supreme Leader Ayatollah Ali Khamenei said that attacking Syria would be catastrophic for the entire Middle East. "Intervention of foreign and extra-regional powers in a country has no result other than sparking fire," he said. "Waging a war is like a spark in a gunpowder store ... its dimensions and consequences can't be predicted."
In Israel, large crowds lined up at gas-mask distribution centres, amid speculation that Syria could hit Israel in response to any Western strike. The Israeli postal service, which oversees gas mask distribution, said demand has tripled in recent days. About five million Israelis now have masks.