During a visit to the site on 17 December, its investigators found “remnants of two RBK-250 PTAB 2.5M cluster bombs, as well as evidence that high-explosive air-dropped bombs were used in the attack”, HRW said.
The air strike had been carried out in the area by Gen Haftar’s Libyan National Army (LNA) or its foreign allies, who include the United Arab Emirates and Russia, HRW said.
There were no reported casualties because that area in southern Tripoli had already been largely abandoned by residents because of heavy fighting.
The use of cluster bombs is banned under an international convention signed in 2008.
Although Libya is among 75 countries – including Russia and the US – which never signed up to the convention, most usually comply with the ban because of the indiscriminate effect of cluster bombs and the long-term danger they pose.
They typically explode in the air and send dozens, even hundreds, of small bomblets over an area the size of a football field.
“Cluster submunitions often fail to explode on initial impact, leaving duds that act like landmines,” HRW said.
Meanwhile, the UN Security Council has adopted a resolution “demanding” that a “lasting ceasefire” comes into force in Libya.
The oil-rich state has been hit by instability since the overthrow of long-time leader Mummar Gaddafi in 2011.
Gen Haftar’s forces launched an offensive last year to capture Tripoli, but have not yet done so.
Turkey has sent troops to bolster the Tripoli-based government.