U.S. Defense Secretary Leon Panetta said on Wednesday the situation in Syria appeared to be "spinning out of control" as he and his British counterpart warned that the Syrian regime ?would be held responsible for securing its chemical weapons.

"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."

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. ?

Still, Western powers have been loathe to intervene militarily in the conflict.

Panetta discussed Syria with British Defense Secretary Philip Hammond, who, speaking at the same Pentagon news conference, said the situation in Syria was "deteriorating and is becoming more and more unpredictable."

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.

Panetta warned that Syria would be held accountable for safeguarding any chemical weapons it possessed.

"We've made very clear to them that they have a responsibility to safeguard their chemical sites and that we will hold them responsible should anything happen with regards to those sites," Panetta said, adding that the United States was working closely with its allies on the issue.