BEIRUT, Lebanon — The Syrian military shelled rebel targets in urban enclaves on Thursday as it readied assault troops and armored columns for a possible invasion of Aleppo, Syria’s densely populated commercial capital, where insurgents have embedded themselves over the past week in preparation for a battle.

Antigovernment activists reached by phone and Skype in Aleppo said that the city’s civilian population was gripped by foreboding as government forces massed on the southern outskirts, and that fierce street clashes had sporadically erupted. But Syrian military commanders appeared to be awaiting reinforcements before issuing invasion orders.

Military experts have long speculated that President Bashar al-Assad’s army, which has been scrambling to crush rebel resistance in urban areas like Homs, Hama and more recently central and southern neighborhoods of Damascus during the uprising, lacked the military resources to take on an armed rebellion in all major cities at once. That seemed to explain the delay in Aleppo, where anticipation of an attack has been building for days.

The United States expressed alarm about the possibility of mass civilian casualties in Aleppo, a Unesco World Heritage site and one of the Middle East’s most storied cities. Victoria Nuland, a State Department spokeswoman, told reporters in Washington that there was “concern that we will see a massacre in Aleppo, and that’s what the regime appears to be lining up for.”