Prime Minister David Cameron says he has discussed the situation in Syria with President Obama and says both agreed that the use or threat of chemical weapons was completely unacceptable and would force them to revisit their approach so far.
Mr Cameron also spoke with French President Francois Hollande about Syria and the Eurozone.
A Downing Street spokesperson said the main focus of both conversations was the appalling situation that continues in Syria.
They agreed that there was much more to do in order the stop the brutal killing of civilians and help put Syria on a path towards peace and stability.
"As with Hollande, the Prime Minister and Obama discussed how to build on the support already given to the opposition to end the appalling violence in Syria and bring about stability. Both said that they wanted to see a credible opposition and hoped that the opposition would use their upcoming meeting in Cairo to show real unity of purpose and coherence in working towards transition.
“Both agreed that the use – or threat – of chemical weapons was completely unacceptable and would force them to revisit their approach so far," the spokesperson said.
They also talked about the desperate plight of Syrian refugees and the Prime Minister emphasised the need to work with the UN and they both agreed that more should be done by the international community to channel humanitarian aid through the UN appeal.
They agreed France’s UNSC ministerial meeting provided a good opportunity to do this.
"The Prime Minister restated the risk to the wider region posed by the fighting and the fact that regional and international cooperation was vital. He reinforced the need to work in close cooperation with Turkey, Qatar, Saudi Arabia and others on the issue.”
The Prime Minister said that he and President Hollande were 100 per cent in agreement as they discussed political, humanitarian and military issues affecting the country and the wider region.
“They discussed how to build on the non-lethal support recently announced by the UK and agreed that France and the UK would 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 Prime Minister and Hollande agreed on the need to encourage other world leaders to maintain international pressure on the regime and support the Syrian people. Both agreed that the refugee situation was deeply troubling and Hollande said that the humanitarian crisis would be the focus of France’s UNSC ministerial meeting at the end of the month."
On the Eurozone, the Prime Minister discussed the issue of Greece with President Hollande. Both welcomed the recent actions of the ECB and agreed that this did not negate the need for Greece to stabilise their own economy and prevent any further detrimental effects to the wider Eurozone.
He said President Obama also welcomed the ECB announcements.