Syrian President Bashar Assad made his first appearance since a bomb killed some of his top lieutenants, looking calm and composed on state TV even as his forces turned parts of Damascus into combat zones and rebels seized two of the country's border crossings.

The unprecedented attack on Assad's inner circle Wednesday, along with the government's inability to crush the rebels after five days of intense clashes in the Syrian capital, point to an unravelling of his grip on power after 16 months of violence.

"It is a war going on here, literally a war," said a 25-year-old woman in the Muhajereen neighbourhood. The sounds of battle had kept her up all night and she stayed home from work because she feared random gunfire, she added.

"It reminded me of that night when the Americans shelled Baghdad nine years ago," said the woman, who spoke on condition of anonymity because she feared for her safety. "I was watching it on TV, but today I'm living a very similar situation."

Even though Assad's powerful military remains mostly loyal — suggesting a total collapse may not be imminent — the rebels appeared to be making startling gains as the civil war intensified.

Besides the fighting in Damascus, about a half-dozen rebels took over a Syrian border crossing near the Iraqi town of Qaim, said Iraqi army Brig. General Qassim al-Dulaimi. There are four major border posts with Iraq.

Rebels overtook a Syrian army outpost near the Syrian-Iraq border after clashes that killed 21 Syrian soldiers, he added.

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This image made from amateur video on Thursday purports to show a Syrian rebel stomping on a picture of President Bashar Assad at the Bab al-Hawa border crossing with Turkey. (Shaam News Network via AP video)

In addition, amateur video posted online showed rebels taking over the Bab al-Hawa border crossing with Turkey, where they stomped on portraits of Assad. The Associated Press could not independently verify the video because the government bars most media from working independently in the country.

A diplomatic solution to ending the bloodshed seemed even more remote after Russia and China again vetoed a Western-backed UN resolution aimed at pressuring Assad's government to end the escalating conflict.

Analysts said the regime was clearly shaken by the violence in the heart of its power base of Damascus, but the next step was not clear yet.

"We should not get carried away with speculating about the impending fall of the regime," said Salman Shaikh, director of the Brookings Doha Center and an analyst on regional politics. He said the regime's forces "are still showing a certain amount of cohesiveness in battle."

Citing a network of sources on the ground, the Britain-based Syrian Observatory for Human Rights reported intense clashes in a string of neighbourhoods along the southern edge of Damascus, the northeastern neighbourhood of Qaboun, and in number of western suburbs.

Chaos in Damascus

Sasa Petricic, a Middle East correspondent with CBC, said there seems to be a great deal of chaos in Damascus itself.

"There are many suburbs and areas of the centres as well that are really war zones right now," he reported from Jerusalem. "The government forces have ... surrounded many of the government buildings. There are snipers on the roofs. But there is a pitched battle that is going on in many parts of Damascus. And many people have been told that they should be leaving the centre within a matter of 48, 36 hours because things will only get worse."

Gunfire and booms from shelling could be heard throughout the capital, and streets in the hard-hit areas were largely empty, save for government troops or rebels.

On Thursday, many Syrians said they were not waiting around to see if the violence would end any time soon. Thousands streamed across the Syrian border into Lebanon at the Masnaa crossing point — about 40 kilometres from Damascus.

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Syrians cross into Lebanon at the border crossing in Masnaa, Lebanon, about 40 kilometres from Damascus, Syria, Thursday. Private cars as well as taxis and buses carried thousands of people fleeing violence in the Syrian capital. (Associated Press)

Hundreds of private cars as well as taxis and buses ferried people across.

However, Israeli Defence Minister Ehud Barak, said Israel would stop Syrian refugees from entering the Israeli-controlled Golan Heights if they try to flee there.

Barak was touring the Golan, which Israel captured from Syria in the 1967 Mideast conflict. Israeli defence officials are closely monitoring the deteriorating situation in Syria, their neighbour to the north, worried it could spill across the frontier.

Barak observed that Syrian refugees, who have already started fleeing to Turkey and Jordan, might also start streaming toward the Golan.

"If we have to stop waves of refugees, we will stop them," he said in the statement.

Past efforts to block crowds trying to storm the Golan have turned deadly. Twice in 2011, hundreds of pro-Palestinian protesters tried to enter the territory from Syria, touching off battles with Israeli security forces.

Syrian situation concerns Israel

The deteriorating situation in Syria has become a mounting concern for Israel, which fears that the collapse of a central regime would give Lebanese Hezbollah militants an opportunity to raid Syrian military arsenals, which include chemical weapons.

Even if Assad did leave power, the opposition is widely perceived to be far too disorganized to take over. There is no clear candidate to lead the country in Assad's absence, and the grim sectarian tint to much of the violence suggests any power vacuum will usher in a bloodbath.

Sunnis make up most of Syria's 22 million people, as well as the backbone of the opposition. But Assad and the ruling elite belong to the tiny Alawite sect, an offshoot of Shiite Islam.

Assad is relying heavily on his Alawite power base to crush the uprising, prompting revenge attacks and fear among other minorities that they face retribution if the regime falls.

Another fearsome factor is the emergence of extremists among the forces looking to oust Assad. Several big suicide attacks this year suggest that al-Qaeda or other terrorist forces are joining the fight.

U.S. officials said al-Qaeda's presence has risen slightly, with one official putting the estimate at a couple hundred operatives attempting to hijack Syrian unrest. They operate under the name of the Al-Nusra Front, which has claimed responsibility for a series of bomb attacks on regime targets, including a government-run TV station in June, the officials said. But opposition forces, such as the leaders of the rebel Free Syrian Army, have made clear to U.S. officials that they reject the group's methods, because the attacks often occur in public areas where civilians have been killed and injured.

The officials spoke on condition of anonymity because they were not authorized to describe the intelligence and analysis publicly.

Assad has not spoken about the bombing that killed three top aides: Defence Minister Dawoud Rajha, 65, a former army general and the most senior government official to be killed in the uprising; Gen. Assef Shawkat, 62, the deputy defence minister who was married to Assad's elder sister, Bushra, and was one of the most feared figures in the inner circle; and Hassan Turkmani, 77, a former defence minister.

Rebels claimed responsibility, saying they targeted the room where the top government security officials in charge of crushing the revolt were meeting.

Assad's appearance Thursday was carried on state TV without audio, but the report appeared aimed at sending the message that he is alive and well, and in control. The report said Assad, wearing a blue suit and tie, swore in the new defence minister and wished him good luck. The report did not say where the ceremony took place.

Rumours swirl about bomb that killed 3 top officials

Assad does not appear in public frequently, and rumours swirled that there was far more to Wednesday's bombing than meets the eye.

It is still a mystery how anybody could have planted a bomb in such a high-level meeting, inside the National Security building, at a time of deep crisis.

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A bomb attack killed three senior members of Syria's ruling regime, including from left to right: former defence minister Hassan Turkmani, Defence Minister Gen. Dawoud Rajha, and Bashar Assad's brother-in-law Major General Assef Shawkat. (SANA/Associated Press)

Some observers believe it was almost certainly an inside job, carried out by a sleeper agent who had been a trusted regime confidante. Others speculate the bombing was actually the work of the regime itself, to decapitate a group of leaders who may have been planning a coup.

Adding to the confusion, Syria's state news agency warned citizens that gunmen were disguising themselves in military uniforms to carry out attacks.

"Gunmen are wearing Republican Guard uniforms in the neighbourhoods of Tadamon, Midan, Qaa and Nahr Aisha, proving that they are planning attacks and crimes," it said.

In one chilling online video, dozens of dead and wounded were shown on the floor of a mosque in the Sayida Zeinab area, south of Damascus. An off-camera narrator says government helicopters fired on them Wednesday. Another video showed what appeared to be at least 40 bodies wrapped in cloth in a mass grave.

The AP could not verify the video.