WASHINGTON: Syria is “rapidly spinning out of control,” US Defense Secretary Leon Panetta said Wednesday, hours after a suicide bombing in Damascus killed three top Syrian security officials.

“It's obvious that what is happening in Syria represents a real escalation in the fighting,” Panetta told reporters in Washington alongside British counterpart Philip Hammond, who agreed with the Pentagon chief's assessment.

Panetta reiterated US demands for President Bashar al-Assad to step down immediately and allow a peaceful transition, accusing him of having ignored repeated appeals from the international community to stem the violence.

He was speaking hours after a suicide attack in Damascus killed at least three top security officials, the first time in the 16-month anti-regime uprising that a bombing has targeted members of Assad's inner circle.

Warning that the situation was “rapidly spinning out of control,” Panetta said the international community must “bring maximum pressure on Assad to do what's right, to step down and to allow for that peaceful transition.”

Hammond said he “absolutely” agreed with Panetta's assessment, adding that the violence was getting “closer and closer to the heart of the regime.”

”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,” Hammond said.

Several Gulf countries are believed to be supplying weapons to the Syrian rebels, often through the Turkish border.

Panetta also stressed that the Assad regime was responsible for ensuring that Syria's chemical weapons were safe and said the United States was working with its allies to ensure that sensitive sites were properly secured.

Both the Assad regime and the opposition “need to understand that the international community is determined to see an orderly and peaceful transfer of power in Syria, not least because of the significance of the presence of these chemical weapon stocks in the country which we do not want to see exposed to a situation that is out of control,” Hammond said.

Syrian officials said the bomber struck as ministers and security officials were meeting at the heavily guarded National Security headquarters in Damascus.

Defense minister General Daoud Rajha, Assad's brother-in-law Assef Shawkat and the head of the Syrian regime's crisis cell, General Hassan Turkmani, were confirmed dead. Interior Minister Mohammed al-Shaar and General Hisham Ikhtiyar, head of National Security, were wounded, according to state media.

Syria's rebel Free Syrian Army (FSA) claimed responsibility for the deadly attack. The brazen assault “just emphasizes the importance of the implementation of the Annan plan,” Hammond said, referring to a peace deal by UN-Arab League envoy Kofi Annan, who is calling on the Security Council to order “consequences” for any failure to carry out his six-point plan.

British diplomats said Annan has asked the UN Security Council to delay a vote originally scheduled for Wednesday on a Western-backed draft resolution calling for sanctions against Syria.

The attack on regime insiders came as battles raged across Damascus and after the FSA, comprising defected soldiers and civilians who have taken up arms against Assad's forces, warned the government to “expect surprises.”

More than 17,000 people have been killed in the uprising, according to regime opponents.