BEIRUT — Syrian rebels are making significant advances in their battle against government forces, raising new questions about President Bashar Assad’s ability to hold on to power and adding urgency to the quest by the international community for a unified and effective political opposition that could take control should his regime collapse.

In the past week, the rebels have seized five important military facilities in the north, the east, and near the capital, Damascus, capturing sizeable quantities of weaponry, further isolating remaining government positions, and freeing up rebel forces to concentrate on attacking them.