Updated

The bombing that killed at least two top Syrian defense officials is evidence that violence there is "rapidly spinning out of control," and it underscores the urgent need for a political solution, U.S. Defense Secretary Leon Panetta said Wednesday.

Appearing with Panetta at a Pentagon news conference, British Defense Minister Philip Hammond said the escalating violence indicates that the rebels feel emboldened and that the government of President Bashar Assad is suffering "probably some fragmentation around the edges" as it struggles to keep a grip on power.

"There is a sense that the situation is deteriorating and is becoming more and more unpredictable," Hammond said.

A bomb ripped through a high-level security meeting Wednesday in the Syrian capital, killing some top regime officials, including Assad's brother-in-law.

Syrian state-run TV confirmed the deaths of Defense Minister Dawoud Rajha, a former army general and the most senior government official to be killed in the rebels' battle to oust Assad; and Gen. Assef Shawkat, the deputy defense minister and one of the most feared figures in Assad's inner circle. He is married to Assad's elder sister, Bushra. An authority with direct knowledge of the situation told The Associated Press that Hassan Turkmani, a former defense minister, also was killed.

The bombing was the harshest blow to the government's inner circle in the 16-month uprising.

Hammond and Panetta both stressed the urgency of finding a political solution that results in Assad's exit.

"It's extremely important that the international community, working with other countries that have concerns in that area, ... bring maximum pressure on Assad to do what's right to step down and to allow for that peaceful transition," Panetta said.

A U.N. Security Council vote is scheduled Wednesday afternoon on a new resolution that would impose sanctions on Syria's government aimed at ending the country's 17-month civil war. But Russia, a longtime Syria ally, is at odds with the U.S. and its European allies on a new resolution that would impose sanctions on Syria's government.

"The violence there has only gotten worse, and the loss of lives has only increased," Panetta said, "which tells us that this is a situation that is rapidly spinning out of control." He said that is all the more reason for the international community to bring "maximum pressure" on Assad to step down and permit a stable transfer of power.

Hammond suggested that Russia and China hold the key to finding a peaceful solution.

"The regime exists at the moment because it receives tacit support from other powers in the world," he said. "If those powers are sending clear messages about the limits of their tolerance for the activities of the regime, that will be an effective constraint on the activities of the regime."

Panetta and Hammond both cautioned the Assad regime not to lose control of its chemical weapons.

"We will not tolerate the use or the proliferation of those chemical weapons," Hammond said, adding, "So our diplomacy has to focus on getting those who have the greatest influence with the regime to ensure that it acts responsibly in relation to chemical weapons."