The Middle East Newsline reported on the assertion, in which HRW identified the weapon as the Chinese-origin Type-81 122 mm rocket.
HRW says Hezbollah fired the cluster weapon during a rocket attack July 25 in the northern Israeli town of Mughar. HRW it was the first confirmed use of such a weapon anywhere in the world.
Israel's military confirmed the HRW report and said 113 cluster munitions were fired into the Jewish state, Middle East Newsline reported.
"We are disturbed to discover that not only Israel but also Hezbollah used cluster munitions in their recent conflict, at a time when many countries are turning away from this kind of weapon precisely because of its impact on civilians," said Steve Goose, director of Human Rights Watch's Arms Division.