(Reuters) - A bomber killed three of Bashar al-Assad's top military officials on Wednesday - including his powerful brother-in-law - in a devastating blow to the Syrian leader's inner circle as rebels closed in vowing to "liberate" the capital.
Slain brother-in-law Assef Shawkat was one of the principal figures in the tight, clan-based ruling elite that has been battling to put down a 16-month rebellion against four decades of rule by Assad and his father.
The defense minister and a senior general were also killed and other top security officials wounded in the attack on a crisis meeting of top Assad security aides that took place as battles raged within sight of the nearby presidential palace.
A security source said the bomber was a bodyguard entrusted with protecting the closest members of Assad's circle. State television said it was a suicide bomb. Two anti-Assad groups claimed responsibility.
The government vowed to retaliate, and residents said army helicopters fired machine guns and in some cases rockets at several residential districts. Television footage showed rebels storming a security base in southern Damascus.
By nightfall, activists said Syrian army artillery had begun shelling the capital from the mountains that overlook it.
Assad's own whereabouts were a mystery - he did not appear in public or make a statement in the hours after the attack. The White House said it did not know where the Syrian leader was.
Diplomacy moved into overdrive as countries spoke of the conflict entering a decisive phase. Washington, which fears a spillover into neighboring states, said the situation seemed to be spinning out of control. Russia's Foreign Minister Sergei Lavrov said "the decisive fight" was under way.
The U.N. Security Council put off a scheduled vote on a Syria resolution. U.S. President Barack Obama spoke with Russia's Vladimir Putin, who has acted as Assad's main protector in the diplomatic arena.
State television said Shawkat and Defense Minister Daoud Rajha had been killed in a "terrorist bombing" and pledged to wipe out the "criminal gangs" responsible. It later said General Hassan Turkmani, a former defense minister and senior military official, had died of his wounds, while Intelligence chief Hisham Bekhtyar and Interior Minister Mohammad Ibrahim al-Shaar were wounded but were "stable".
The men form the core of a military crisis unit led by Assad to take charge of crushing the revolt which grew out of a popular protests inspired by Arab Spring uprisings that unseated leaders in Tunisia, Egypt and Libya.
"I heard ... a loud explosion but it was not a very big bang. I went down to take a look and I saw a lot of men in plain clothes with rifles," one resident near the scene told Reuters by telephone. Windows on the third floor of the national security building were shattered.
Security sources said Assad was not at the meeting where the attack took place. The armed forces chief of staff, Fahad Jassim al-Freij, quickly took over as defense minister to avoid giving any impression of official paralysis.
"This cowardly terrorist act will not deter our men in the armed forces from continuing their sacred mission of pursuing the remnants of these armed terrorist criminal gangs," Freij said on state television. "They will cut off every hand that tries to hurt the security of the nation or its citizens."
The explosion appeared to be part of a coordinated assault on the fourth day of fighting in the capital that rebel fighters have called the "liberation of Damascus" after months of clashes which activists say have killed more than 17,000 people.
It began early on Wednesday with fighting around an army barracks in the district of Dummar, hundreds of meters from the presidential palace, and was followed by blasts close to the base of the elite 4th armored division in the southwest. The unit, led by Assad's brother Maher, has been instrumental in crushing protests around Syria.
Assad's enemies described victory as imminent.
"This is the final phase. They will fall very soon," Abdelbasset Seida, leader of the opposition Syrian National Council, told Reuters in Qatar. "Today is a turning point in Syria's history. It will put more pressure on the regime and bring an end very soon, within weeks or months."
U.S. Defense Secretary Leon Panetta said: "This is a situation that is rapidly spinning out of control." He called for maximum global pressure on Assad to step down.
Panetta said Assad's government would be held responsible if it failed to safeguard its chemical weapons, which Western and Israeli official have said have been moved from storage sites.
Kofi Annan, the former U.N. Secretary-General who has acted as a peace envoy but whose calls for a ceasefire have fallen on deaf ears, said world powers should act to halt the bloodshed.
"DECISIVE BATTLE"
A video posted by activists who said it was filmed in the southern Qadam district showed at least two bodies lying in pools of blood and one rebel commander said at least 45 civilians had been killed in Damascus on Wednesday.
There was no way to confirm the figure, and he gave no tally of rebel or security forces casualties. The Syrian government restricts access by international journalists.
Western leaders fear the conflict, which has been joined by al Qaeda-style Jihadists, could destabilize Syria's neighbors - Israel, Lebanon, Turkey, Iraq and Jordan.
Syrian Information Minister Omran Zoabi blamed Western and Sunni Arab governments for the crisis. "They are responsible for every drop of blood. And they will be accountable," he said.
"I stress to them that this is the decisive battle in all of Syria," Zoabi said on state television.
Rebels say they have brought reinforcements from outside the city to topple Assad by attacking the power base of the ruling elite for the first time.
Syrian forces hit rebel positions across the capital after the attack on the security meeting, with activists saying government troops and pro-government militia were flooding in.
State television broadcast footage it said was filmed on Wednesday showing men in blue army fatigues ducking for cover and firing - the first time official media has shown clashes in the heart of the capital.
REBEL CLAIMS
Two rebel groups claimed responsibility for the attack on the security meeting.
"This is the volcano we talked about, we have just started," said Qassim Saadedine, a spokesman for the Free Syrian Army, a group made up of army defectors and Sunni youths.
Liwa al-Islam, an Islamist rebel group the name of which means "The Brigade of Islam", said it had carried out the attack by planting a homemade bomb in the building.
Fighting also erupted overnight in the southern neighborhoods of Asali and Qadam, and in Hajar al-Aswad and Tadamon - poor, mainly Sunni Muslim districts housing Damascenes and Palestinian refugees.
Assad and the ruling elite belong to the minority Alawite sect, an offshoot of Shi'ite Islam whose power was cemented after a coup in 1970. The elite has endured more than a year of rebellion but recent high level defections have signaled support beginning to fall away.
Two Syrian brigadier-generals were among 600 Syrians who fled from Syria to Turkey overnight, a Turkish official said on Wednesday, bringing the number of Syrian generals sheltering in Turkey to at least 20.
In Damascus, government troops used heavy machine guns and anti-aircraft guns against rebels moving deep in residential neighborhoods, armed mostly with small arms and rocket-propelled grenades.
Rebel fighters have called the intensified guerrilla attacks in recent days the battle "for the liberation of Damascus".
Still, some opposition figures did not predict easy victory.
"It is going to be difficult to sustain supply lines and the rebels may have to make a tactical withdrawal at one point, like they did in other cities," veteran opposition activist Fawaz Tello said from Istanbul.
"But what is clear is that Damascus has joined the revolt."
