Syrian antigovernment activists said Wednesday that government forces and insurgent factions in the Damascus suburbs planned to announce a Russian-brokered deal for a 15-day cease-fire that could begin as soon as Thursday. United Nations officials and international negotiators are trying to hammer out a countrywide cease-fire as world leaders gather in Vienna in an effort to renew peace talks, and it was unclear whether the Russian-brokered effort was separate and parallel, or a step toward reaching a more comprehensive agreement. The Russian involvement was seen as a sign that it now had the upper hand — more so than Iran, President Bashar al-Assad’s closest ally. The truce would cover the East Ghouta suburbs that have been cut off from Damascus by the fighting. The truce would open checkpoints into East Ghouta, allowing humanitarian aid to get in and civilians to leave, according to several antigovernment activists reached in the area.