AMMAN/BEIRUT: Syrian rebels clashed with forces loyal to President Bashar al-Assad in Damascus, while across much of the city streets were deserted and houses and shops shuttered for fear of violence after Wednesday's killing of three close Assad allies.
Efforts to forge a diplomatic solution appeared to collapse when Russia and China vetoed a U.N. Security Council resolution that threatened Syrian authorities with sanctions if they did not stop using heavy weapons and pull troops from towns.
In Damascus, residents said the city appeared paralysed after the killing of Assad's brother-in-law, defence minister and a top general in a bomb attack on a security meeting. Some districts suffered heavy shelling.
Syrian TV flashed a warning on its screen, telling residents that gunmen disguised in Republican Guard uniforms were spreading through several of Damascus's troubled districts, saying "they are planning to commit crimes and attack people."
Activists also issued counter warnings, saying real Republican Guard forces were in Midan. "We tapped into their walkie-talkies ... we are afraid of a massacre," said activist Samir al-Shami.
Assad, who had no made no public statement or appearance since the stunning bombing attack on a crisis meeting of his defence and security chiefs, was shown on Syrian television on Thursday at the swearing in of his new defence minister.
"Everyone is looking now at how well Assad can maintain the command structure. The killings yesterday were a huge blow, but not fatal," said a Western diplomat following Syria.
Residents said there was no let-up in the heaviest fighting - now in its fifth day - to hit the Syrian capital in a 16-month revolt against Assad. His family has dominated for 42 years the pivotal Arab country bordering Israel, Lebanon, Jordan and Iraq.
Western officials fear rising unrest in Syria, which some have called a civil war, could spill across its borders.
Residents said a heavy onslaught of security force shelling and firing from helicopters went through the night and continued on Thursday in Damascus. Some reported explosions in the troubled north-eastern and southern districts of the capital.
A witness said rebels attacked the main police headquarters in Damascus. "Gunfire has been intense for the past hour. It is now dying down but the streets around the police command remain empty," said a resident of Qanawat, an old central distric where the Damascus Province police headquarters is located.
Areas without fighting were largely deserted. Residents said the roads to many southern districts where fighting was heaviest were closed or peppered with checkpoints.
The streets of the city centre were nearly empty and there was no trace of the rebels, even as the sound of fighting could be heard in several areas. Most shops were shuttered.
But in areas where there was fighting, some of it leaving a trail of bloody corpses in the streets, residents were nervous and confused.
"Everyone in the neighbourhood is arming themselves. Some with machineguns, some with shotguns. Some even just with knives," said one resident near the troubled Midan district.
"I can't even tell you what is going on outside because I've shuttered the windows and locked the doors. I just hear every now and then the gunfire, it's like it's in the room," said one resident who lived near the flashpoint Midan district.
A Western diplomat said Sweden, Denmark and Austria had pulled out diplomats temporarily from Damascus after Wednesday's bombing. The three missions had already been operating with a limited staff with only one or two diplomats each on the ground. Most of them hoped to return next week, he said.
Russian Foreign Minister Sergei Lavrov said on Wednesday "the decisive fight" was under way in Damascus.
Russia's vetoing of a Security Council resolution to impose international sanctions on Assad was the third such blocking vote by moscow. The Kremlin, which has backed the Damascus administration, argues that sanctions unfairly target Assad while applying no pressure on the rebels.
The bombing on Wednesday seemed part of a coordinated assault on the capital that has escalated since the start of the week. Rebels call it the "liberation of Damascus" after months of fierce clashes which activists say have killed 17,000 people.
A security source said the bomber who struck inside the security headquarters was a bodyguard for Assad's inner circle. Anti-Assad groups claimed responsibility.
U.N. Secretary-General Ban Ki-moon condemned the Damascus blast and expressed grave concern over the use of heavy weapons.
"Time is of the essence," he said. "The Syrian people have suffered for too long. The bloodshed must end now."
Western leaders also worry that the conflict in the heart of the Arab world has been joined by al Qaeda-style jihadists.
U.S. Defence 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.
A woman who visited the neighbourhood of Tadamon, a scene of heavy fighting, said a police station was destroyed.
"I saw five charred bodies strewn across the street," she told Reuters by telephone. "Seven police cars were torched ... some mosques in Damascus are calling on loudspeakers which shelters are available for people who have fled."
Syria TV said footage of the fighting published by activists and aired by television outlets was staged. It is difficult to verify activist reports as the government restricts foreign media access.
People fleeing violent areas began searching for safe havens, some even taking refuge in the marble courtyards of the ancient Umayyad Mosque in Damascus's historic Old City.
"We tried to find cheap hotels but all of them were full with other people who fled. So we came here to the mosque and said look, we have nowhere to go, nowhere to sleep," one woman in the group told a resident who toured the area.
While fighting rages in Damascus, clashes and shelling have also continued across the country.
Rebels said they had "liberated" the town of Azaz in northern Aleppo province, bordering Turkey. Activists also published video of the town of Talbiseh, in central Homs province, being sprayed with gunfire from helicopters above.
Fighting also erupted near the Syrian-Israeli frontier, and Israel promptly responded saying it would not accept refugees.
"In the event of the regime's downfall, which could happen... (Israeli forces) here are alert and ready, and if we have to stop waves of refugees, we will stop them," Defence Minister Ehud Barak said.