Syrian clashes hone in on crucial city of Aleppo as Damascus violence flares NEW: The regime denies reports of helicopter attacks and says Damascus is "normal" NEW: At least seven people are killed across Syria on Sunday, an opposition group says A rebel leader announces an operation "to liberate Aleppo" from "Assad thugs" Aleppo is the largest city and the commercial hub of Syria (CNN) -- After 16 months of bloodshed, the Syrian crisis now zeroes in on Aleppo, the country's most populated city and a critical one for both President Bashar al-Assad's regime and rebels. In a video posted online Sunday, the head of the rebel Free Syrian Army in Aleppo announced an operation "to liberate the city of Aleppo from the rule of the Assad thugs whose hands were blood stained by heinous crimes against our people." Brig. Gen. Abdel Jabbar Al-Obeidi urged regime soldiers to defect or step aside and not fight against his men, with the promise that no one would be harmed. He vowed to secure Aleppo and protect all minorities and sects, including the members of Alawite sect that the president belongs to. Meanwhile, a mosque in the capital of Damascus came under attack from rocket and helicopter shelling, leading to "many" injuries, an opposition group said. The mosque caught fire, and "families remain trapped within the disaster-stricken neighborhood" of Barzeh, the Local Coordination Committees of Syria said. But the Syrian regime denied reports of helicopter attacks in Damascus, calling them "baseless." "The situation in Damascus is very normal, and life is as usual but in a limited number of neighborhoods," Syrian state-run TV reported. Opposition groups said the chaos wasn't limited to Damascus; at least seven people were killed across the country Sunday, the LCC said. Aleppo and the capital city of Damascus have been al-Assad strongholds. But heavy clashes in Damascus over the past week, along with recent bombing that killed four members of al-Assad's inner circle and government, suggest cracks in the regime's armor. Fierce clashes broke out Sunday morning between regime and rebel forces in Aleppo, opposition activists said. "An entire building collapsed in Seif al-Dawla neighborhood after a regime tank ... targeted it with four shells," the Local Coordination Committees of Syria said. If rebels gain control of Aleppo, the commercial hub of Syria, it would mark a pivotal point in the Syrian crisis and deal a heavy blow to al-Assad's financial ties. In May, a violent crackdown on a peaceful demonstration at Aleppo University was one of the turning points of the uprising, with anti-regime students and civilians shot in the streets. Meanwhile, an opposition group reported Saturday that a senior scientist and expert in Syria's missile program was killed, along with his wife and son. Retired Maj. Gen. Nabil Zugheib was assassinated in the Bab Touma district in Damascus, the Syrian Observatory for Human Rights said. There was no immediate claim of responsibility. Thousands of Syrians race to borders Damascus is now largely isolated by checkpoints and tanks, witnesses said Saturday, with tanks, artillery and mortars pummeled the neighborhood of Barzeh. At least 13 of 140 people killed across the country Saturday were in Damascus or the Damascus suburbs, the LCC said. The crisis started in March 2011, when a fierce government crackdown against protesters morphed into a nationwide uprising against the regime. The United Nations estimates more than 10,000 people have been killed since the crisis began more than 16 months ago. But Rupert Colville, a spokesman for the office of the U.N. high commissioner for human rights, said the United Nations has not been giving out overall death toll numbers since December "because it became impossible to verify the numbers in any meaningful way." The LCC, for example, estimates that more than 16,000 -- mostly civilians -- have died. Military defections have hit the Syrian regime. An official from the Turkish Foreign Ministry said two brigadier generals from Syria arrived in Turkey on Friday night and one arrived the night before. Diplomats have yet to come up with a plan to effectively stop what is now being called a civil war. The Arab League is expected to meet Sunday in Doha, Qatar, to discuss Syria, according to a senior Arab League official. The secretary-general will meet first with a five-member committee handling Syria before presenting a proposal to the wider group, also Sunday night, the official said. Why Syria could get even uglier CNN's Ivan Watson and Yesim Comert contributed to this report.