Britain, France and the United States are discussing how to bolster the Syrian opposition, which is fighting government forces 17 months into a popular uprising against Syrian President Bashar al-Assad's rule.
British Prime Minister David Cameron and US President Barack Obama also agreed that the use or threat of chemical weapons use by Syria was "was completely unacceptable" and would force them to "revisit their approach" to the conflict.
"As with (French President) 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," Cameron's office said in a statement.
The White House said the telephone call covered a "wide array of global issues," including the conflict in Syria, and the need for increased participation from other countries to support the opposition to the Assad government.
Obama conveyed his concerns about the "increasingly dire humanitarian situation in Syria," and the need for contributions to humanitarian appeals in the region, the White House added.
Obama also reaffirmed his support for decisive action by European leaders to stabilise the financial crisis in the euro zone, according to the White House.