ANTAKYA, Turkey – Syrian rebels seized control of several critical border crossings Thursday as thousands of people fled the escalating violence in the capital, offering fresh evidence that the regime led by President Bashar al-Assad is starting to unravel.

Street fighting stretched into a fifth day in Damascus, with government soldiers deploying snipers on rooftops and helicopter gunships in flash-point neighborhoods.

More than 20,000 people were reported to have fled into neighboring Lebanon, and activist groups said more than 55 were killed in Damascus and its suburbs a day after a bombing in the heart of the city killed three of Assad’s most senior advisers.

Russia and China vetoed a U.N. Security Council resolution that would have imposed sanctions on Syria, effectively heralding an end to diplomatic efforts to resolve the crisis. And the latest violence seemed to leave little doubt that both sides are gearing up for a fight to the finish.

“There’s no way the armed opposition would go for a negotiated settlement now,” said Jeffrey White, defense fellow at the Washington Institute for Near East Policy. “They believe they are winning, and I think they probably are.”

Wednesday’s bombing in a key security building in an upscale Damascus neighborhood shattered any illusions that the inner circle of power in the country was impregnable and left the regime looking shakier than at any point in the 16-month-old uprising.

Rumors that Assad also had been killed were quelled when the president made his first appearance since the attack in a brief segment broadcast on state TV on Thursday showing the swearing in of the new defense minister, Fahd Jassim al-Freij.

The pictures depicted the two men standing in a room with an ornate chandelier and tables but did not mention a location, further fueling speculation that Assad may have left Damascus.

Seizures symbolic

It was unclear whether the seizure of border crossings into Turkey and Iraq would provide a significant boost to the rebels’ efforts to oust the Assad regime. When rebels briefly seized a Syrian post on the Turkish border a few weeks back, Turkey simply closed the crossing.

But it offered a powerfully symbolic reminder of the government’s vulnerability in the outlying provinces of the country, which have long been in open revolt and where armed rebels hold sway over large regions.

“These operations are important because they boost the morale of our people and are political messages,” Col Malik Kurdi, the deputy commander of the Free Syrian Army, said by telephone from the military refugee camp in southern Turkey where the nominal rebel leadership is based.

Free Syrian Army fighters overran the main Abu Kamal border crossing between Syria and Iraq about 8 p.m. Thursday, tearing down pictures of Assad, burning the Syrian flag and erecting the revolutionary one, according to Farhan Fteikhan, the mayor of Qaim district in the adjoining Anbar province in Iraq.

The seizure was a problem for Iraq’s Shiite-led government, which has offered hesitant support to Assad for fear that the ascendancy of the Sunni-dominated insurgency in Syria would encourage its own Sunni opponents.

Iraq imposed a curfew on the Qaim area to prevent infiltration by insurgents on either side of the border and has dispatched 1,500 soldiers to the border area, Fteikhan said.

Similar scenes unfolded Thursday at the Bab Hawa crossing point on the Turkish border. But a rebel on the border who asked to be identified by his alias, Mutassim al-Sarmadawi, later said the rebels withdrew from the post Thursday night after threats by government forces to launch a major attack on the nearby village of Sarmadi.

The incident illustrated the seesawing nature of the conflict in Syria’s countryside, where towns routinely change hands every few weeks and neither side has been able to gain a decisive advantage. That may change after the bombing, by emboldening soldiers who had previously been afraid to defect.