The UK has joined the US in warning Syria that the use or threat of chemical weapons would force them "to revisit their approach".
The warning came after a telephone call between Prime Minister David Cameron and US President Barack Obama.
The PM also spoke to French President Francois Hollande. The three discussed building support for the opposition.
Earlier, Chinese state media accused Mr Obama of using the chemical arms issue as an excuse for military intervention.
Also on Wednesday, fierce fighting raged in the northern Syrian city of Aleppo and in Damascus, which residents said had witnessed the heaviest attack by government forces since the army re-asserted its control of the capital last month.
A Downing Street spokesman said the "appalling situation that continues in Syria" was the main focus of Mr Cameron's conversations with Mr Hollande and Mr Oba
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