British Prime Minister David Cameron is urging the United Nations Security Council to pass “clear and tough” messages on sanctions against Syria.

The Council is scheduled to vote Thursday on a Western-backed draft resolution that could impose sanctions on President Bashar al-Assad's government.

Mr. Cameron appealed to countries that have resisted previous calls for sanctions — a reference to Russia and China — asking what additional evidence they need about a “brutal” government. He also said it is time for Mr. Assad to go.

Following growing violence and Wednesday's bombing that killed top Syrian officials, U.S. Secretary of Defense Leon Panetta said the situation in Syria is spinning out of control and urged the international community to pressure Mr. Assad to step down.

Russia condemned the bombing as an act of terrorism and said it hopes the masterminds will be found and punished.