The regime of Syrian President Bashar Assad is coming under increased pressure as the civil war descends on Damascus.
Cameron spoke by video conference with U.S. President Barack Obama on affairs in the Middle East and Central Asia. Their talks come amid growing support for a Syrian opposition council formed last month in Qatar, which both sides recognized.
Both leaders "agreed that the Assad regime was under increasing pressure and that it was important that the international community continue efforts to force a political transition, to support the emergence of a more coherent and effective opposition and to relieve the humanitarian suffering," a statement from Cameron's office read.
The pro-Western March 14 coalition in neighboring Lebanon called for more support for the Syrian opposition given the growing support in the Arab community.
There are concerns about the security of Syria's chemical weapons stockpile given the fighting in Damascus. Victoria Nuland, a spokeswoman for the U.S. State Department, said the Assad regime is obligated to keep its weapons secure.
"The president holds the regime responsible for the security of the chemical weapons of Syria," she said.