Fresh fighting in many parts of Syria throughout the night has followed the deaths on Wednesday of three top regime figures in a suspected suicide attack.
Syrian government and opposition both said large numbers of people died, in one of the bloodiest days of the conflict.
Activists said artillery and helicopters were used in the worst attack, on a funeral south of Damascus.
The president’s brother-in-law, defence minister and head of Bashar al-Assad’s crisis team died in yesterday’s bombing.
Rebel groups said the bomb had been planted the day before the meeting at national security headquarters where it was detonated. They predicted the government’s imminent fall.
The army has pledged to rid Syria of “criminal and murder gangs”.
The London-based Syrian Observatory for Human Rights said on Thursday that more than 150 people had died across the country on Wednesday, in one of the worst days of a 16-month revolt.
Syrian government and opposition both said large numbers of people died on July 18, in one of the bloodiest days of the conflict
Video of one attack posted on the internet showed scenes of pandemonium after what activists said was a helicopter gunship attack on a funeral procession at Sitt Zeinab, south of the capital. They said at least 60 people were killed in this incident alone.
In Damascus, state media said, security forces launched operations in many areas which have been clashes in recent days, mainly in the south-west and north-east, killing many “terrorists”.
Activists reported more tanks moving towards the capital from the west.
Following Wednesday’s bombing, the government has vowed to root out ruthlessly what it describes as armed terrorists backed by outside powers.
The rebels are on the offensive too, warning state TV and radio to evacuate their personnel before its headquarters comes under attack.
In contrast with earlier explosions in Damascus, there were no photos or video from the scene of Wednesday’s blast at the security headquarters.
Among the victims were:
• Defence Minister and ex-chief of staff Gen Daoud Rajiha
• Deputy Defence Minister Assef Shawkat, married to Bashar al-Assad’s sister Bushra
• Assistant to the vice-president and head of crisis management office Gen Hassan Turkomani
• Two other senior officials – interior minister Mohammad Ibrahim al-Shaar and National Security Bureau chief Hisham Ikhtiar- were wounded
The US said the killings were a major blow to the regime.
“I think the incident today makes clear that Assad is losing control,” White House spokesman Jay Carney told reporters.
“All of our partners internationally need to come together to support a transition.”
In Lebanon, Hezbollah leader Hassan Nasrallah condemned what he termed a targeted killing: “We are going to miss them and we offer our condolences to the Syrian leadership and the Syrian army.”
Russia said some countries had incited the opposition rather than calming it down.
The attack prompted the UN Security Council to delay until Thursday a vote on a Western-sponsored resolution calling for tougher sanctions on Damascus.
UN Secretary General Ban Ki-moon said the Security Council must “shoulder its responsibility and take collective and effective action.
“Time is of the essence. The Syrian people have suffered for too long. The bloodshed must end now,” Ban Ki-moon said.
The mandate for the UN’s observer mission runs out on Friday. The resolution before the Security Council would extend the mission and place international envoy Kofi Annan’s peace plan under Chapter 7 of the UN Charter, which could ultimately authorize force.
But Russia is firmly against harsher measures. In a telephone conversation between President Barack Obama and President Vladimir Putin on Wednesday, the two leaders were said to be divided in their approaches to ending the bloodshed.
Syria’s defense minister Daoud Rajiha and his deputy Assef Shawkat, President Assad’s brother-in-law, have died in a suspected suicide bombing at security headquarters in Damascus, state TV says.
Daoud Rajiha and Assef Shawkat were attending a meeting of senior officials at the time.
The national security chief and interior minister are said to be critically hurt.
The attack comes amid claims of a major rebel offensive on the city.
A statement by the armed forces read out on TV shortly after the attack 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.
The rebels now clearly believe that victory is within sight, and the deaths will give them even greater heart.
Security sources say the suspected bomber worked as a bodyguard for members of President Bashar al-Assad’s inner circle.
General Daoud Rajiha has been defense minister for less than a year, serving previously as chief of staff, and is on a US blacklist for his role in the suppression of dissent.
He is believed to be an Orthodox Christian – a rarity in the Alawite-dominated Syrian military and government.
General Assef Shawkat is considered a top security chief and a member of the inner circle of the regime. He is married to Bashar al-Assad’s sister Bushra.
General Assef Shawkat was a top security chief and Bashar al-Assad’s brother-in-law
In other developments:
• Turkish officials report two Syrian generals are among hundreds of refugees who fled into Turkey overnight, bringing the total number of fleeing generals to 20.
• Turkish Prime Minister Recep Tayyip Erdogan is expected to discuss the Syrian crisis with Russian President Vladimir Putin.
The area around the national security building, in Rawda district, has been sealed off.
Witnesses at the site of the bombing said journalists were banned from approaching.
“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,” the TV said.
The reports say that Hisham Ikhtiar, director the National Security Bureau, and Interior Minister Mohammad Ibrahim al-Shaar, were among those injured in the attack.
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, said the entrances to Damascus were closed on Wednesday morning.
“We heard the sounds of explosions every now and then all around Damascus,” she said.
“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.
“People are trying to run away and get out of al-Qaboun.”
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 Free Syrian Army (FSA) rebels, and said the barracks is 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.
But the TV carried night-time footage of troops deployed in the Midan quarter, in some very tense and deserted streets.
The rebels have declared a final battle for the capital, calling it Operation Damascus Volcano, and have been fighting troops in several parts of Damascus for the past three days.
The fighting reached central areas on Tuesday, with gunfire and plumes of smoke reported in a street near parliament.
The Free Syrian Army said the operation was well planned, and they had sent hundreds of fighters to the capital last week to be in place for the assault.
The rebels and the government often publish contradictory accounts of the same incidents.
Western journalists are under heavy restrictions in Syria, making it difficult to verify the claims of either side.
The attack comes as UN chiefs have been trying to persuade China and Russia to agree tougher measures on Syria, ahead of a Security Council vote on Wednesday on imposing sanctions.
UN-Arab League envoy Kofi Annan held talks with Russian President Vladimir Putin in Moscow, and UN Secretary General Ban Ki-moon met Chinese leader Hu Jintao in Beijing.
The UN has until Friday to renew the mandate for observers in Syria, although a vote is expected in New York on Wednesday afternoon.
Western nations want a new resolution threatening measures short of the use of force.
The Western-backed draft resolution to be discussed gives the Syrian government 10 days to withdraw heavy weapons from cities and return troops to barracks, otherwise a further resolution on sanctions will be submitted to the Security Council.
With Russia resisting all efforts to persuade it to take a tougher line with Syria, there is virtually no hope of concerted international action to pull the country back from the brink.
About 16,000 people have died in Syria since protests against the Assad regime began in March last year.