Rebels pushed their "Damascus Volcano" assault Thursday after three senior security chiefs were killed in a bombing Washington said indicated President Bashar al-Assad was losing control of Syria.
The deaths Wednesday of Defence Minister General Daoud Rajha, Assad's brother-in-law Assef Shawkat and General Hassan Turkmani, head of the regime's crisis cell, came on one of the deadliest days in the 16-month conflict.
More than 200 people, mostly civilians, were killed, including 38 in the capital as rebels pressed their all-out offensive, the Syrian Observatory for Human Rights said.
The explosion, blamed on a bodyguard attending a meeting at a Damascus security headquarters, was the first targeting Assad's inner core since the uprising erupted in mid-March last year.
It came ahead of a showdown on Thursday between the West and Russia and China over a draft UN resolution calling for sanctions against Syria.
UN chief Ban Ki-moon and international envoy Kofi Annan called on the Security Council to take strong action, but Russia and China are expected to veto the resolution.
Ban said there was an "extreme urgency" for action to halt the violence which activists say has killed more than 17,000 people.
Annan, the UN-Arab League envoy, said the council must take "decisive" action after he persuaded the major powers to postpone the vote originally set for Wednesday.
"The deteriorating situation in Syria underscores the extreme urgency for all sides to stop armed violence in all its forms, implement the six-point plan and move swiftly towards a political dialogue," Ban said in a statement.
Annan "urged members of the Security Council to unite and take concerted and strong action that would help stem the bloodshed in Syria and build momentum for a political transition," said his spokesman Ahmad Fawzi.
Conflicting accounts have emerged of Wednesday's attack that also wounded Interior Minister Mohammed al-Shaar and General Hisham Ikhtiyar, head of National Security.
It was claimed by the rebel Free Syrian Army as its fighters battled Assad loyalists across Damascus for a fourth straight day, although another group, the Brigade of Islam, also said it was responsible.
The FSA command said the attack targeted officials "responsible for barbaric massacres."
The rebels said the attack, part of Operation Damascus Volcano launched on Monday, "is the first in a series... aimed at bringing down Assad and the pillars and symbols of the regime, whether civilian or military."
State media initially said it was a "suicide bombing" before apparently retracting and calling it a "terrorist attack."
A Syrian security official told AFP the bombing was carried out by a bodyguard wearing an explosives belt, but another official said it was caused by a briefcase packed with explosives that a bodyguard detonated by remote control.
Rajha, a Christian, was the highest-ranking officer in the army under Assad's overall command.
The Syrian Observatory called Shawkat's death "a severe blow to the Syrian regime since he played the main role in operations by regular forces to crush the revolution."
Assad, whose whereabouts are not known on Thursday, appointed General Fahd al-Freij as new defence minister -- who promptly appeared on television vowing to "hunt down criminal gangs and cut off the hand of those who threaten security."
The blast came after the FSA declared its battle to "liberate" Damascus had begun and warned the regime to "expect surprises."
At least 214 people -- 124 civilians, 62 soldiers and 28 rebels died on Wednesday in one of the bloodiest days of the revolt, the Britain-based Observatory said, revising an earlier toll.
That figure did not include the three members of Assad's inner circle.
White House National Security Council spokesman Tommy Vietor said Assad was "losing control," pointing to "increasing" defections and a "strengthened and more united" opposition.
Pentagon chief Leon 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."
Russian President Vladimir Putin agreed Wednesday in a phone call with US President Barack Obama following the attack to work with the United States to find a solution to the crisis, the White House said.
But the Kremlin stressed that "differences" remain ahead of the UN Security Council vote.
Russia earlier demanded the arrest and strict punishment of those behind the bombing that it called an "act of terror."
"We see the events as another attempt to further destabilise the situation in Syria," the foreign ministry said, calling on both sides to reassess the situation and seek peace.
Jordan's King Abdullah II warned meanwhile that his northern neighbour was on the brink of all-out civil war and that in a worst-case scenario, Syria's chemical weapons could fall into the "unfriendly" hands of Al-Qaeda.
He told CNN the Damascus bombing was a "tremendous blow" to Assad but not yet the death knell for a regime that remains determined to cling to power.