David Cameron has joined with the US President in warning Syria that the use of chemical weapons in Syria or even a threat to deploy them would be "completely unacceptable".
In a phone call the two leaders said if President Bashar al Assad made such a move it "would force them to revisit their approach so far", Downing Street said.
The Prime Minister and Barack Obama said there was "much more to do" to stop the brutal killing of civilians in the Middle East state.
Earlier this week Mr Obama warned that any movement of Syria's stockpile of chemical weapons would be a "red line" which would have "enormous consequences".
Mr Cameron also discussed the situation with French President Francois Hollande and the two European nations agreed to "work more closely to identify how they could bolster the opposition and help a potential transitional Syrian government after the inevitable fall of Assad".
The leaders said they hoped a meeting of opposition groups in Cairo would show "real unity of purpose and coherence in working towards transition".
Mr Cameron and Mr Obama agreed that more should be done by the international community to channel humanitarian aid to Syrian refugees through the UN appeal.
Meanwhile Russia has accused Western powers of "openly instigating" Syrian opposition groups to take up arms in their fight to unseat Mr Assad.
Russia's Foreign Ministry said the West "has done nothing" to urge the Syrian opposition to start a dialogue with the government.
"Instead, they are engaged in openly instigating it to continue their armed struggle," it said in a statement.
Russia and China, both veto-wielding UN Security Council members, have blocked proposals to call on Mr Assad to step down.
