Damascus - In the space of two days, some of President Assad's inner circle of power, including his brother-in-law, have been killed, an army barracks overlooking the presidential palace has been engulfed in flames, and clashes have been moving closer to the heart of the capital.

It doesn't necessarily mean the end is imminent, as opposition circles and armed rebels on the ground clearly believe.

The battle for Damascus has barely begun. If it remains a purely military affair, the regime still has a daunting superiority in weaponry and manpower.

But with international diplomacy effectively paralysed, the pressure within the crisis seems to be building up, to an intensity where more surprises may be expected. Security sources say the suspected bomber worked as a bodyguard for members of President Bashar al-Assad's inner circle.

As events in Damascus unfolded, a UN Security Council vote on a Western-sponsored resolution threatening Syria with tougher sanctions was postponed until Thursday following a request by UN and Arab League envoy Kofi Annan.

Condemning the violence, Mr Annan urged members of the Security Council to take strong, concerted action to help stem the bloodshed.

"The terrorist explosion which targeted the national security building in Damascus occurred during a meeting of ministers and a number of heads of [security] agencies," state TV said.

The BBC's Lina Sinjab in Damascus says none of the windows of the building appears to be broken. There is no sign of extra security, she adds.

Gen Daoud Rajiha had been defence minister for less than a year, serving previously as chief of staff, and was on a US blacklist for his role in the suppression of dissent.

He was believed to be an Orthodox Christian - a rarity in the Alawite-dominated Syrian military and government.

Gen Assef Shawkat was married to Mr Assad's sister Bushra and considered a top security chief and a member of the inner circle of the regime.

He is the closest person to the president to be killed so far and his loss is a triple blow for the ruling family, BBC security correspondent Frank Gardner says.

Gen Hassan Turkomani was a former defence minister and assistant to the vice president as well as being in charge of President Assad's crisis management office.

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The Syrian government's information minister Omran Zoab, said the attack was cowardlyA long-standing senior member of the ruling Baath party and a Sunni Muslim, unlike many in the Syrian elite, he was put in charge of the security forces' crisis team when the uprising began in 2011, opposition activists said.

Hisham Ikhtiar, director of the National Security Bureau, and Interior Minister Mohammad Ibrahim al-Shaar, were among those hurt in the attack, state TV said.

Witnesses at the site of the bombing, in Rawda district, said journalists were banned from approaching.

The defence minister has been replaced by Gen Fahd Jassim al-Furayj, chief of staff of the armed forces, state TV reports.