Fierce fighting broke out in the Syrian capital, Damascus, as rebels attacked forces loyal to President Bashar al-Assad, witnesses and rebels said.
Much of the violence was centred around the Jobar district and a key junction on the Damascus ring road.
But it was not clear whether the rebels managed to retain any territory they claimed in battle, and the Syrian army said it too launched an offensive.
Earlier, reports said bombings in the town of Palmyra killed 19 people.
The violence in both Damascus and Palmyra came as the head of the main Syrian opposition alliance, Moaz al-Khatib, said his offer of talks with the government would be withdrawn unless women prisoners were released by Sunday.