The UK has joined the U.S. in warning Syria that the use or threat of chemical weapons would force them "to revisit their approach", BBC News reports.

The warning came after a telephone call between Prime Minister David Cameron and U.S. 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 Obama of using the chemical arms issue as an excuse for military intervention.

Also on Wednesday, Aug 22, 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 Cameron's conversations with Hollande and Obama.

Mr Cameron and Mr Obama both agreed "that the use - or threat - of chemical weapons was completely unacceptable and would force them to revisit their approach so far", said the spokesman.

The comments echoed those by Obama earlier in the week, when he said he would change his thinking on intervention if Syria used chemical weapons.

The two leaders, along with Hollande, discussed "how to build on the support already given to the opposition" and "help a potential transitional Syrian government after the inevitable fall of [President Bashar al-] Assad".

Obama and Cameron called for a "credible opposition" that would "show real unity of purpose and coherence in working towards transition".

The three leaders also discussed the plight of Syrian refugees.

The spokesman said: "The prime minister emphasized the need to work with the UN and... that more should be done by the international community to channel humanitarian aid through the UN appeal."