The rights group said Syria used "a powerful type of cluster munition rocket not seen before in the conflict" in an attack last week on the town of Keferzita, north of the restive city of Hama.
Human Rights Watch said the munition, an antipersonnel fragmentation device, contains more than 395 bomblets inside, some which have the same mass as a 9mm bullet.
"It is appalling that Syrian government forces are still using banned cluster munitions on their people," Steve Goose, arms division director at Human Rights Watch, said in a statement from Washington. "Cluster bombs are killing Syrian civilians now and threatening Syrians for generations to come."
The rights group said at least two people were killed and another 10 were wounded in the attacks.
Rebel forces took control of Keferzita in late 2012. The rights group said the town was the frequent target of airstrikes and other bombardments since then.
Rival groups drove elements of al-Qaida from the area last month and Human Rights Watch said there are no remaining rebel targets left in the region.