Syrian forces on Monday counter-attacked rebels in the capital, Damascus, a day after the opposition fighters staged a surprise assault on government-held neighborhoods in the northeastern part of the city.
Witnesses reported seeing pro-government warplanes active over Damascus, though it was not clear if they were Syrian or from ally Russia, which has been fighting in support of President Bashar al-Assad.
Monday's clashes included the Jobar neighborhood, where on Sunday opposition fighters seized several buildings as part of an offensive that began with two car bombings and several suicide attacks.
The rebels have lost territory to pro-government forces to the south and west of Damascus during the past year, but still hold some areas to the east of the city. The main site of their gains during the same period has been in northwestern Syria, along the border with Turkey, where opposition fighters backed by Turkey have pushed out Islamic State militants.
The conflict in Syria has been going on for more than six years and has been complicated by the competing aims of pro-Syrian fighters, multiple rebels groups, Kurdish forces and Islamic State.
The United Nations is working to jump-start peace talks to bring an end to the war, with the next round of discussions planned for later this week.