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Meanwhile, Barack Obama described Syria’s alleged chemical weapons attacks as “troublesome” and of “grave concern” on Friday, as Britain went a step further by directly blaming the Assad regime.

The cautious tone of the president’s first public response to the reported gassing of hundreds of people in Damascus on Wednesday morning suggested that U.S. policy would not change. Obama stressed the risks attached to “costly interventions” abroad.

However, William Hague, the British Foreign Secretary, blamed the Syrian government, saying: “We do believe this is a chemical attack by the Assad regime on a large scale.”

The use of toxic gas was the only “plausible explanation for casualties on this scale in such a small area”, he told Sky News.

France has threatened military action if it is proven the Syrian government used chemical weapons against its own people.

A team of United Nations experts is in Damascus to investigate three earlier events in which poison gas was allegedly used. But they can move only with the consent of the authorities and have not received permission to visit the locations of the latest attacks.

“The fact that Syria won’t let the UN inspectors who are on the ground in the country today visit this area is a very telling action,” Baird said.

The UN’s senior disarmament official is to arrive in the capital on Saturday and will push for access to the reported attack site for the observers, Ban Ki-moon, the UN secretary-general, said yesterday.