Syrian rebels seized control of several critical border crossings Thursday as thousands of people fled the rapidly 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.

With the veto by Russia and China of a U.N. Security Council resolution aimed at Syria effectively heralding an end to diplomatic efforts to resolve the crisis, the spiraling 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 killed Assad’s defense minister, Dawoud Rajha; the head of his crisis management cell, Hassan Turkmani; and his brother-in-law Assef Shawkat. The attack 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.

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 swaths of territory.

“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 in a telephone interview 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.

Seven border posts were also burned, he said, and there was no attempt to resist by Syrian forces, suggesting they had either abandoned their posts or defected to the opposition.

The seizure represented 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. Iraqi troops are also restoring barbed-wire fences that were erected by U.S. forces and then dismantled when they left.

Similar scenes unfolded Thursday at the Bab Hawa crossing point on the Turkish border, with rebels seen tearing down Assad posters and firing AK-47s into the air in a video posted online.

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 Wednesday’s bombing, by emboldening soldiers who had previously been afraid to defect.

Sarmadawi said a group of about 40 soldiers escaped from the Bab Hawa customs post and approached the rebels late Wednesday, saying they wanted to defect and would help the rebels take over the crossing point.

“ ‘We swear to God we wanted to defect before, but we couldn’t,’ ” Sarmadawi quoted the soldiers as saying. “ ‘But our commanders are now in chaos. They have lost control.’ ”

When fighters reached the post the next day, the remaining soldiers either joined the rebels or slipped away.

But the incident also underscored the regime’s capacity to bring overwhelming force to bear against the lightly armed guerrillas when it chooses, making for what could be a prolonged battle across the country.

“There are obvious signs that the regime is cracking, but I don’t think things will just tip over,” said Andrew Tabler of the Washington Institute for Near East Policy. The government has not brought all its resources into play and could make greater use of combat helicopters, start using fixed-wing bombers and resort to the use of chemical weapons.

“There are several ways it can accelerate,” Tabler said.

With Assad’s grip slipping even in Damascus, there was a growing sense that it may be only a matter of time before his regime crumbles.

The capital has begun to resemble a battleground, with fighting between rebels and pro-government forces reported in many Damascus neighborhoods Thursday. Army snipers spread across rooftops in the Mezze area, and helicopters were targeting rebel fighters in the neighborhood of Barzeh, according to the Revolution Leadership Council of Damascus, an opposition group.

Government forces were also shelling several outlying neighborhoods as a sense of fear spread across the city. Many stores were closed, and there were sporadic blackouts and Internet outages.

“I have been a prisoner at home for the last three days,” Abed, a Qaboun resident, said. “I’m left with no bread and three bags of frozen vegetables.”

Reuters reported that about 20,000 Syrians had crossed the border into Lebanon in the past 24 hours. Wael Khalidi, the director of a nongovernmental organization called Syrian Refugees Affairs, said he thought the number was much lower. “We are afraid the Lebanese authorities will stop allowing refugees in and we will be in big trouble,” he said.

Dehghanpisheh reported from Beirut. Uthman al-Mukhtar in Fallujah, Iraq, Suzan Haidamous and Ahmed Ramadan in Beirut, Jabbar Yaseen in Baghdad and a special correspondent in Damascus contributed to this report.