"An investigation that looks only at the military's own records is hardly a thorough investigation," HRW's executive director Kenneth Roth was quoted as saying in the statement.
In November, HRW provided Ukrainian prosecutors with information on nine indiscriminate and thus unlawful attacks carried out in the Donetsk area.
Circumstances suggest that government forces were responsible for the shelling, the organization said.
"Before denying government responsibility, investigators should examine other independent evidence to determine who used these indiscriminate weapons," Roth said.
The organization also stated that investigators had examined the use of the wrong cluster weapon. While the military prosecutor concluded that government forces had not carried out cluster munition attacks in Donetsk, investigators had not checked the inventory of the 9M27K rocket "that was actually used in Donetsk in early October".
In addition to conducting a full investigation, HRW urged the Ukrainian authorities to issue explicit orders to troops in the field, banning the use of Grad rockets in populated areas and the overall use of cluster munitions, even when firing in response.
"Violations by one side cannot be used to justify violations by another side," Roth underlined.
A bloody conflict has raged in Ukraine's eastern regions since spring, when Kiev authorities launched a military operation aimed at curbing the region's pro-independence movement.
