British Prime Minister David Cameron called on Thursday on Syria's president to step down following an escalation of violence and bombings in the capital Damascus.

Cameron also appealed to Russia and China to support a new U.N. resolution that threatens non-military sanctions and is tied to Chapter 7 of the U.N. Charter, which could eventually allow the use of force to end the conflict.

"It is time for the United Nations Security Council to Pass clear and tough messages about sanctions, I believe under Chapter 7 of the U.N., and be unambiguous about it," Cameron said.

Russia, which is a close Syrian ally, has said it will veto any Chapter 7 resolution.

In Moscow on Wednesday, Russia's Foreign Minister Sergey Lavrov pointed to Wednesday's deadly bombing in the heart of Damascus that killed the defense minister and his deputy, President Bashar Assad's powerful brother-in-law, and accused the West of inciting the Syrian opposition.