BEIRUT/AMMAN (Reuters) - Mystery surrounded the whereabouts of Syrian President Bashar al-Assad on Thursday, a day after a bomber killed and wounded his security chiefs and rebels closed in on the centre of Damascus, vowing to "liberate" the capital.
The Syrian leader made no public appearance and no statement on Wednesday after a bomber killed his powerful brother-in-law, his defense minister and a top general.
By the early hours of Thursday morning, residents had reported no let-up in the heaviest fighting to hit the capital in a 16-month revolt against Assad's rule.
Fighting on Wednesday had come to within sight of the presidential palace, near the security headquarters where the bomber struck a crisis meeting of defense and security chiefs.
Assad's brother-in-law Assef Shawkat, who served as a top commander and one of the pillars of the Assad clan's rule, was killed in the blast along with Defense Minister Daoud Rajha.
Another senior general also was killed and the heads of intelligence and the Interior Ministry were wounded, deeply damaging the security apparatus of the Assad family, which has ruled the country with an iron fist for four decades.
Intense clashes were reported late on Wednesday in the capital's central districts of Mezze and Kafar Souseh, while a police station in the Hajar al-Aswad district was in flames.
The army was shelling its own capital from the surrounding mountains as night fell. Government troops, having vowed retaliation on Wednesday for the assassination, fired machineguns into the city from helicopters.
A security source said the bomber who struck inside the security headquarters was a bodyguard entrusted with protecting the closest members of Assad's circle. State television said it was a suicide bomber. Anti-Assad groups claimed responsibility.
Washington, which fears a spillover into neighboring states, said the situation seemed to be spinning out of control. Russian Foreign Minister Sergei Lavrov said "the decisive fight" was under way in Damascus.
The U.N. Security Council put off a scheduled vote on a Syria resolution on Wednesday and U.S. President Barack Obama phoned Russia's Vladimir Putin, Assad's main world power protector, to try to persuade Moscow to drop support for him.
The generals killed and wounded in Wednesday's bombing form the core of Assad's crisis unit to crush the revolt, which grew out of protests inspired by Arab Spring uprisings that unseated leaders in Tunisia, Egypt and Libya.
The armed forces chief of staff, Fahad Jassim al-Freij, quickly took over as defense minister on Wednesday 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 capital that has escalated since the start of the week. Rebel fighters call it the "liberation of Damascus" after months of clashes which activists say have killed 17,000 people.
"This is the final phase. They will fall very soon," Abdelbasset Seida, a 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.
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 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.
Government troops used heavy machineguns and anti-aircraft guns against rebels in residential neighborhoods, armed mostly with small arms and rocket-propelled grenades.
Rebels were jubilant at their success in penetrating into the capital and at the deaths of the security chiefs. Abdullah al-Shami, a rebel commander based in Turkey, said: "I expect a speedy collapse of the regime ... and it means we will not be in need of outside intervention, with the regime beginning to crumble much faster than we envisaged."
Yet some opposition figures said victory would still not be easy.
"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."
Additional reporting by Dominic Evans and Erika Solomon in Beirut, Khaled Yacoub Oweis in Amman, Suleiman Al-Khalidi in Antakya, Turkey and Regan Doherty in Doha; Writing by Peter Graff; Editing by Louise Ireland