Activists reported that violence had spilled into areas that had usually been calm. A mortar shell landed on Baghdad Street, a downtown thoroughfare, killing a 22-year-old man, said Salam Mohammed, an activist in Damascus. There were clashes at Mezze Airport, west of the capital, near a wealthy pro-government area that had usually been isolated from fighting, another activist said.

Emile Hokayem, an analyst at the International Institute for Strategic Studies, described the fighting as “part of the strategy of encirclement of the city.”

“The rebels are making a very strong point: that they can go after anything that is seen as critical infrastructure,” Mr. Hokayem said, adding that while the government would be able to reopen the airport and the airport road, “the cost of doing so is only going to increase over time.”

Fighting has plagued the Damascus suburbs throughout the 20-month conflict, and rebels have tried several times to push into the city. Most recently, they held the southern neighborhood of Medan for several days over the summer. The government responded by moving forces from other areas to Damascus, analysts said, and there were indications that it had also done so in recent days. Activists reported that government forces had withdrawn from one of their last bases in the remote eastern province of Deir al-Zour, leaving rebels in control of oil fields there.