A bomb struck at the heart of Syria's senior command on Wednesday, killing at least three of President Bashar al-Assad's top brass in an attack claimed by rebels who warned of more carnage to come.
The attack, which an official blamed on a bodyguard attending a meeting of security chiefs at their Damascus headquarters, prompted the White House to say Assad was "losing control" of his country.
The blast which targeted Assad's inner core for the first time in a 16-month uprising came ahead of a showdown between the West and Russia and China over a draft UN resolution calling for sanctions.
That vote appeared to have been delayed by a day to Thursday.
The blast killed defence minister general Daoud Rajha, Assad's brother-in-law Assef Shawkat and General Hassan Turkmani, head of the regime's crisis cell on the uprising, state media said.
Among those wounded were interior minister Mohammed al-Shaar and General Hisham Ikhtiyar, head of national security, the television said.
Conflicting accounts have emerged of who carried out the attack and how.
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 "announces the good news of the outstanding operation this morning that targeted the National Security headquarters and the killing" of the officials "responsible for barbaric massacres," it said.
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 of one of the ministers or security chiefs at the meeting. The attacker had been wearing an explosives belt.
Another official said the blast was caused by a briefcase packed with explosives that a bodyguard left in the meeting room and detonated from afar via remote control.
The FSA said in a YouTube video that six people died including Interior Minister Shaar, General Hisham al-Bikhtyar, head of the national security branch, and Mohammed Saeed al-Khitan, identified as "deputy local trustee."
Rajha, a Christian, was the highest-ranking officer in the army under Assad's overall command.
The Syrian Observatory for Human Rights 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," notably in flashpoint Homs province.
State media said Assad appointed General Fahd al-Freij as new defence minister, and he promptly appeared on television vowing to "hunt down criminal gangs and cutting off the hand of those who threaten security."
The Syrian army said the "terrorist act increases the armed forces' determination to cleanse the country of terrorist groups."
But news of the bombing was greeted by wild celebrations in Jabal Shahshabu, a rebel stronghold in central Syria where people fired Kalashnikov assault rifles into the air and shouted "Allahu Akbar" (God is greatest).
The attack came a day after the FSA -- comprising army defectors and armed civilians -- declared its battle to "liberate" Damascus had begun and warned the regime to "expect surprises."
'Assad losing control'
Columns of black smoke rose over Damascus as troops shelled Qaboon and Barzeh and fighting raged across the city on Wednesday, activists said.
The Local Coordination Committees activist network reported fighting in Al-Midan and Zahira districts, and loud explosions in the western suburb of Mashrou-Dumar.
Outside the capital at Jdaidet Artuz, blasts and the sound of gunfire could be heard in the area, where electricity supplies had been cut off.
White House National Security Council spokesman Tommy Vietor said there was growing momentum against Assad, 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."
France said "Assad must understand that his struggle to retain power is futile and that nothing will stop the Syrian people's march to a democratic future."
Russia demanded the arrest and strict punishment of those behind what 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.
Moscow had earlier given notice it would not back the Western-drafted UN resolution on the crisis.
"Now the Damascus Volcano, the battle for the capital and a decisive battle have been declared in Syria. Adopting the resolution would mean outright support of a revolutionary movement," Foreign Minister Sergei Lavrov said.
In contrast, Britain said Syria was tipping into chaos and collapse, and that a strong UN stand was needed to push for a transition.
"This incident, which we condemn, confirms the urgent need for a Chapter VII resolution of the UN Security Council on Syria," said Foreign Secretary William Hague.
UN chief Ban Ki-moon urged the council to unite and take action on the "very serious" situation after meeting President Hu Jintao of China, which has twice joined Moscow to block resolutions condemning Damascus.
The current 90-day UN mission in Syria ends on Friday, and if no resolution is passed by then, it would have to shut down this weekend, diplomats say.
According to British officials, UN-Arab League envoy Kofi Annan has asked the Security Council to delay the vote calling for sanctions.
But even so, the United States announced on Wednesday that it was slapping 29 members of Assad's regime with a new set of sanctions of its own.