BEIRUT — Antigovernment activists in Syria said the military fired Scud missiles into at least three rebel-held districts of Aleppo on Friday, flattening dozens of houses, killing at least 12 civilians and burying perhaps dozens of others under piles of rubble.

The assertion, which appears to be corroborated by videos posted on the Internet, came one day after Syrian government targets in central Damascus were hit by multiple car bombings that were among the deadliest and most destructive so far in the nearly two-year-old conflict.