Three men at the heart of President Assad's defence team have died in a suicide bombing, Syrian state TV says.
The president's defence minister, brother-in-law and head of his crisis team were at a meeting at national security headquarters in Damascus.
No footage has yet emerged of the attack in which the national security chief and interior minister were also said to have been wounded.
It comes as rebels claim to have launched an offensive on the capital.
For the past three days, rebels have fought with troops in several parts of the city, declaring their operation, entitled Damascus Volcano, a final battle for the capital.
The rebel Free Syrian Army (FSA) and a jihadist group calling itself Lord of the Martyrs Brigade both said they were behind the security headquarters bombing.
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.
A 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.
An armed forces' statement read out on TV said Syria was "more determined than ever" to fight terrorism and wipe out "criminal gangs".
Whoever thinks that killing top commanders "can twist Syria's arm... is delusional", it said.
But the BBC's Jim Muir, in neighbouring Lebanon, says the rebels now clearly believe victory is within sight, and the deaths will give them even greater heart.
Earlier, activists reported more clashes during the night in several areas around the south-west of Damascus.
They said the government had brought more troops and armour into some districts, and that several people had been killed in clashes and bombardments.
A rebel spokeswoman, Susan Ahmad, told the BBC the entrances to Damascus were closed on Wednesday morning.
"Now tanks are storming into al-Qaboun [district], shelling everything, shelling residential houses, shooting every moving thing and they are trying to arrest people and kill.
Activists have also posted on the internet pictures of what they say is a barracks on the heights overlooking the city engulfed in flames.
They believed it had been hit by fire from FSA rebels, and said the barracks was involved in providing security for the presidential palace complex below.
State media said security forces fought off attacks by small groups of armed terrorists in the city.
Western journalists are under heavy restrictions in Syria, making it difficult to verify the claims of either side.
The UN Security Council had been due to vote on a new round of sanctions against Syria and Russia's Deputy Foreign Minister Gennady Gatilov tweeted that there was a "dangerous pattern" of militant attacks coinciding with Security Council meetings on Syria.
UN chiefs, who have until Friday to renew the mandate for observers in Syria, have been trying to persuade China and Russia to agree tougher measures on Damascus.
President Barack Obama and Russian President Vladimir Putin discussed the crisis by telephone on Wednesday. But, according to Interfax news agency, although they agreed on a final goal they disagreed on how to reach it.
Opposition groups say as many as 16,000 people have died in Syria since protests against President Assad began in March last year.