The western districts held by Syrian President Bashar al-Assad have not experienced the severe deprivations of areas in the east controlled by rebel forces. But after an array of rebels and extremists linked to al-Qaeda broke the brutal government siege of opposition neighborhoods last week, the rebels escalated the assault to besiege the government side. That has disrupted supplies of food and medicine to an area where more than a million people live, potentially testing loyalties of residents to the embattled Syrian leader.

In response, rebels and opposition activists say, Assad’s forces have responded with intensified bombings that have struck hospitals and involved munitions containing chlorine gas, a choking agent. Compounding the misery, U.N. officials said Tuesday that fighting had disabled Aleppo’s main power plant, which had pumped water to 2 million people on both sides of the city.

“Prices are getting expensive, and businessmen are choosing not to sell what they have because they want to profit later when prices get even higher,” said Hisham, a resident of a loyalist district in the city’s west end who asked that his last name not be published because of concerns for his safety. Because of Tuesday’s disruptions, he added, his neighborhood now depends on water that is trucked in.