BEIRUT – International concern was mounting Friday over a potential massacre as Syrian troops bombarded the besieged city of Aleppo with artillery, strafed it with aircraft and reportedly pulled in major reinforcements ready to crush the outgunned rebels.

The battle is one of the most important of the 17-month-old uprising. With a population of about 3 million, Aleppo is Syria's largest city and commercial hub, a key pillar of support for President Bashar Assad's regime.

The rebels controlled several neighborhoods but were facing reports of troops and tanks massing outside the city. The nonstop fighting in Aleppo already has claimed the lives of at least 145 rebels and civilians in the last six days, according to the Britain-based Syrian Observatory for Human Rights.