DAMASCUS - A suicide bomber struck Wednesday at the heart of Syria's top command, killing three of President Bashar al-Assad's security chiefs in an attack claimed by rebels who warned of more carnage to come.
The bombing, which an official blamed on a bodyguard attending a meeting of security chiefs at their Damascus headquarters, prompted US Defence Secretary Leon Panetta to warn that Syria was "spinning out of control."
The attack, 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 now appears to have been pushed back by a day to Thursday.
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.
The blast killed Defence Minister General Daoud Rajha, Assad's brother-in-law Assef Shawkat and General Hassan Turkmani, the head of the regime's crisis cell, state media said.
Among those wounded were Interior Minister Mohammed al-Shaar and General Hisham Ikhtiyar, head of National Security.
The rebel Free Syrian Army claimed the attack, which came as its fighters battled Assad loyalists across Damascus for a fourth straight day.
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."
Defence Minister 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."
State television said Assad had appointed Fahd al-Freij as new defence minister, while the military said the "terrorist act increases the armed forces' determination to clean the country of terrorist groups."
The attack came a day after the FSA -- comprising army defectors and civilians who have taken up arms -- declared that its battle to "liberate" Damascus had begun and warned the government to "expect surprises."
-- 'Syria spinning out of control' --
Columns of black smoke rose over the capital on Wednesday as troops shelled the districts of Qaboon and Barzeh and fighting raged across the capital, activists said.
"Qaboon is surrounded by tanks, and regime forces are shelling the quarter from all sides," the Union of Coordinators of the Syrian Revolution said on Facebook.
The Local Coordination Committees -- a grassroots 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, but still in Damascus province, blasts and the sound of gunfire could be heard in the area, where electricity supplies had been cut off.
Northwest of the city near the border with Lebanon, troops bombarded the town of Zabadani, said the LCC.
In Washington, Pentagon chief Panetta said the situation was "spinning out of control" and that international community must "bring maximum pressure on Assad to do what's right, to step down and to allow for that peaceful transition."
France also said the attack showed the urgent need for a political transition.
"The situation in Syria is worsening day by day as the violence intensifies. Bashar al-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," foreign ministry spokesman Bernard Valero said.
Russia demanded the arrest and strict punishment of those behind what it called an "act of terror."
"We expect the organisers of the act of terror in Damascus to be identified and for them to face their deserved punishment.
"We see the events as another attempt to further destabilise the situation in Syria," the foreign ministry said, calling on both Assad and the Syrian people to reassess the situation and seek a peaceful compromise.
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, British Foreign Secretary William Hague 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 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 critical of 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 diplomats, 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 the Syrian regime with a new set of sanctions of its own.