Washington - The United States concedes that Russia will not allow the UN Security Council to authorise military strikes against the Syrian regime, senior White House officials said on Friday.
Speaking on condition of anonymity, the officials said a UN report due on Monday would strengthen the case that Bashar Assad used chemical weapons against his own people.
But they admitted that this, while weakening Moscow's claim that anti-government rebels fired the poison gas, would not be enough to swing the Kremlin behind military action.
Instead, they warned the United States retains the option of acting unilaterally or in a small coalition if a Russian-led plan to secure and destroy Assad's chemical arms fails.
In the meantime, they said Western powers would try to find tougher sanctions short of force to put in a draft resolution against Syria, in the hope that Moscow drops its veto.
Assad and his ally Russia's President Vladimir Putin allege the 21 August attack - which US intelligence says killed 1 400 people - was carried out by anti-government rebels.
But Washington and Western and Arab powers, backed by independent human rights groups - say the strikes, which hit pro-rebel suburbs - came from government rocket batteries.
United Nations experts are due to report on Monday and, although they are not mandated to assign blame, a senior US official said their evidence would leave Russia more isolated.
Despite their differences, US Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov are working on a plan to take Assad's weapons under international control.
The US officials, who refused to be directly quoted, said the White House expected that it would take around two weeks to discover whether the Russian-led initiative was viable.
US President Barack Obama discussed the issue with Sheikh Sabah al-Ahmad al-Sabah, the emir of Kuwait, at the White House on Friday.
"Our two countries are in agreement that the use of chemical weapons that we saw in Syria was a criminal act, and that it is absolutely important for the international community to respond in not only deterring repeated use of chemical weapons, but hopefully getting those chemical weapons outside of Syria," he said.
"I shared with the Emir my hope that the negotiations that are currently taking place between Secretary of State Kerry and Foreign Minister Lavrov in Geneva bear fruit.
"But I repeated what I've said publicly, which is that any agreement needs to be verifiable and enforceable."
The US officials said a genuine disarmament plan would be preferable to US strikes, but that these have not been ruled out.