The situation in Syria is “spinning out of control,” U.S. Defense Secretary Leon Panetta said on Wednesday, adding that President Bashar al-Assad’s government would be held responsible if it failed to safeguard its chemical weapons sites.

Syria’s defense minister and Assad’s brother-in-law were killed in a Damascus suicide bomb attack carried out by a bodyguard on Wednesday, the most serious blow to the president’s high command in the country’s 16-month-old rebellion.

“This is a situation that is rapidly spinning out of control,” Panetta said, adding that the international community needed to “bring maximum pressure on (President Bashar al-Assad) to do what’s right, to step down and allow for that peaceful transition.”

Panetta’s comments to a Pentagon news conference followed closed door talks with his British counterpart, Defense Secretary Philip Hammond. Hammond, speaking alongside Panetta, said he believed the situation in Syria was deteriorating and “becoming more and more unpredictable.”

The Damascus attack, Hammond said, showed the country’s growing instability as the violence gets closer to the heart of the government.

“I think what we’re seeing is an opposition which is emboldened, clearly an opposition which has access increasingly to weaponry, probably some fragmentation around the edges of the regime as well,” he told reporters.

The Assad government appears to be quietly shifting some chemical weapons from storage sites, Western and Israeli officials have said, but it is not clear whether the operation is merely a security precaution amid Syria’s escalating internal conflict.

The Syrian government denies carrying out the operation. Syrian’s undeclared stockpile - believed to be the largest of its kind in the Middle East - reportedly includes sarin nerve agent, mustard gas and cyanide.