United States Defence Secretary Leon Panetta says the situation in Syria appears to be spinning out of control.
Three key men in President Bashar al-Assad's regime were killed by a bomb at the national security headquarters in Damascus on Wednesday.
Among the dead were Defence Minister Daoud Rajiha and President Bashar al-Assad's brother-in-law, Assef Shawkat, and the head of the crisis management office, General Hassan Turkomani.
Among those wounded were Interior Minister Mohammed al-Shaar and the head of national security, General Hisham Ikhtiyar.
General Rajiha had been defence minister for less than a year. He previously served as chief of staff. General Shawkat was married to Mr Assad's sister.
General Turkomani was a former defence minister, who was put in charge of the security crisis centre when the uprising began in 2011.
Syrian state television says a suicide bomber carried out the attack. Liwa al Islam, a rebel group, has claimed responsibility for the bombing.
Security sources say the suspected bomber worked as a bodyguard for members of President Assad's inner circle.
The area around the building, in the Rawda district, is sealed off.
US officials said they did not have any information on Mr Assad's whereabouts.
At the Pentagon, Mr Panetta said Syria was lurching into dangerous territory.
The White House says the uprising is gaining real momentum and President Assad is losing control of Syria.
US President Barack Obama has spoken with Russian President Vladimir Putin about the crisis.
A European national security official says while the bomb is a huge setback for the regime, it is not necessarily a death blow.
However, the head of the Syrian National Council says the assassination is a turning point that will quicken an end to the conflict.
Abelbasset Seida called it a sign the rebellion has entered its final phase.
The United Nations Security Council has postponed a vote on a resolution that threatens Syrian officials with sanctions.
Russia says the vote will now be held on Thursday, at the request of the UN / Arab League envoy Kofi Annan.
Security post attacked
Arab television stations later broadcast live images of Syrian rebels storming a security post in southern Damascus.
The fighters were seen surrounding the building, firing and waving rifles.
Smoke was rising in the background, though it was not clear whether the building itself was on fire.
There have been three days of heavy fighting in Damascus.