Political uproar in London meanwhile cast doubt on whether Britain will join American military action to attack Syria for an alleged chemical weapons attack should the response take place before next week.

And a team of UN inspectors pressed on with its hazardous work in Damascus, testing victims of the alleged poison gas attack, which killed hundreds of people last week and threatens to draw reluctant Western states into a vicious civil war.

Obama said Washington had definitively concluded that the President Bashar al-Assad's government was to blame for last week's attack.

Washington is due to publicly release a report into the incident that it says will prove its allegations, possibly as soon as Thursday.

Asked how close he was to ordering a US strike, expected to start with cruise missile raids, Obama told PBS NewsHour: "I have not made a decision."

But he warned that US action would be designed to send a "shot across the bow" to convince Syria it had "better not do it again."

The US leader, who wants to seal a legacy of ending foreign wars, not getting into new ones, argued that it was vital to send a clear message not just to Syria, but around the world.

"We do have to make sure that when countries break international norms on weapons like chemical weapons that could threaten us, that they are held accountable."

Earlier, Washington bluntly signaled that a UN Security Council resolution proposed by Britain that could have given a legal basis for an assault was going nowhere, owing to Russian opposition.

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