As many as 30,000 Syrians may have fled across the border to neighboring Lebanon in the past 48 hours, as intense fighting spread across the Syrian capital of Damascus and attacks spiked in other cities across the country.

 “We have reports we are trying to verify that thousands of Syrians overnight and yesterday crossed into Lebanon,” Melissa Fleming, a spokeswoman for the United Nations High Commissioner for Refugees, said at a briefing in Geneva on Friday. “So far reports vary between 8,500 and 30,000 people who may have crossed.”

 With so many Syrians crossing the border in such a short period of time, some Lebanese are concerned that the influx could spark sectarian conflict in their own country, where recent clashes between Sunni Muslims and Alawites have left more than 20 dead.

 Syrian state television reported, meanwhile, that Lt. Gen. Hisham Ikhtiyar, the country’s national security chief, died of wounds sustained Wednesday when a bomb planted by rebels detonated in a meeting of high-level security officials.

 Ikhtiyar was the fourth member of President Bashar al-Assad’s inner circle killed by the explosion, which shattered the already fragile chances of a negotiated peace deal and brought the civil revolt that has raged for 16 months into the heart of Assad’s government.

 Government forces shelled residential areas of Damascus and deployed tanks, armored vehicles and helicopters Friday, claiming they had made headway in their battle against rebel forces by retaking Midan, a neighborhood where some of the heaviest fighting had taken place in recent days. The rebel Free Syrian Army (FSA) claimed it had made a tactical retreat. “We pulled back to prepare other attacks,” said Malik Kurdi, deputy commander of the FSA.

 The Syrian military also regained control of two border crossings between Syria and Turkey that the rebels had taken over on Thursday, though rebel fighters were still in control of another border crossing between Syria and Iraq.

 The heavy fighting on Thursday and Friday left 431 dead across Syria, according to the Britain-based Syrian Observatory for Human Rights, a sign that the conflict may be headed toward a bloody endgame as the government struggles to regain control in Damascus and several smaller cities.

 The U.N. Security Council voted unanimously on Friday to extend the life of the troubled U.N. observer force in Syria for another 30 days, avoiding an abrupt U.N. pullout only hours before the mission’s mandate was set to expire at midnight. The vote followed Thursday’s veto of a resolution that threatened Syria with tough new sanctions. Russia and China, which cast the vetoes, voted in favor of the temporary extension of the unarmed observer mission.

 The resolution approved Friday, which was introduced by Britain, left open the possibility that the mandate of the U.N. mission could be extended longer if the violence subsides and if Syria withdraws its heavy weapons from the country’s restive towns. But the U.S. ambassador to the United Nations, Susan E. Rice, signaled that the fate of the U.N. operation remained in doubt. She said the United States had approved Friday’s action largely to give the blue helmets time to pack up their bags and “withdraw safely and. . .in an orderly fashion.”

 Some members of the opposition are already betting that they’ve got the upper hand and are planning for what comes after Assad. The Syrian Muslim Brotherhood, which is part of a broader coalition of anti-government groups, announced its plans to form an Islamist party. “We are ready for the post-Assad era, we have plans for the economy, the courts, politics,” said Mulhem al-Droubi, the Brotherhood’s spokesman, according to Agence France-Presse.

 While the fighting between government forces and rebel fighters remained largely in neighborhoods on the periphery of the capital in recent days, the clashes on Friday extended into the center of Damascus. Thousands of people began gathering on Khalid bin Walid street, a major thoroughfare leading from the commercial area downtown to the restive Midan district, immediately after Friday prayers and were attacked by government forces. The crowds “started to chant for freedom when the security attacked them, shooting some and injuring others,” said a young woman who witnessed the attack, speaking on the condition of anonymity because of safety concerns.

Clashes continued in the area throughout the afternoon, and rebel fighters blasted the central police headquarters on the street with three rocket-propelled grenades, according to opposition activists. One video posted on YouTube on Friday showed two bodies lying in the center of Khalid bin Walid street as gunfire broke out nearby. Another video allegedly showed the police building on fire.

 In Qaboun, a neighborhood that has been rocked by heavy fighting in recent days, pro-Assad shabiha militiamen reportedly carried out executions, according to anti-government groups, who posted a gruesome video of two men who had been shot in the head and back with their hands tied behind their back.

 With the violence ramping up dramatically, residents were scrambling to get out of the capital, where electrical blackouts and food shortages are now common and uncollected garbage has piled up in the streets.

 Rama, a young Damascus resident who asked to use her nickname for her safety, said she and her family had tried for two days to find a driver to take them to the border. They found a willing driver Friday morning who asked for twice the usual price and they left immediately. “I did not say goodbye to my best friends or to my neighbors,” she said. “We just decided to leave as soon as possible.”

 When the family arrived at the border, they faced a chaotic scene and it took them six hours to cross. Rama, her parents and her two siblings are now packed into a house with her aunt in Beirut, but they have no regrets about leaving.

 “It was a matter of time before the situation escalated and reached our house,” Rama said.

Liz Sly in Antakya, Turkey; Colum Lynch at the United Nations; Ahmed Ramadan in Beirut; Suzan Haidamous in Masnaa; and a special correspondent in Damascus contributed to this report.