"It is outrageous that Kiev and its US patrons are attempting to place the blame for the tragedy in the town of Volnovakha on the militia, without awaiting a trial or investigation," the foreign ministry said in a statement Sunday.
According to the statement, posted on the ministry's website, an investigation report released by the Special Monitoring Mission (SMM) deployed in Ukraine by the Organization for Security and Co-operation in Europe (OSCE) refutes Ukraine's allegations.
The ministry demanded "an immediate and impartial investigation into the tragedy with the participation of all parties concerned".
On Saturday, the SMM published the conclusions of its investigation into the January 13 bus attack that killed 12 people near a Ukrainian military checkpoint close to the government-controlled town of Volnovakha in the eastern Donetsk Region.
The investigation included a comprehensive analysis of two specific blast craters, including one located 98 feet (30 meters) from the side of the bus. The SMM asserted that all the examined craters were caused by rockets fired from the northeast direction.
Commenting on the results of the report, Russian Ambassador to the OSCE Andrei Kelin said they contradict Kiev's claims that the bus was hit by local militia from the east.
The authorities of the self-proclaimed People's Republic of Donetsk (DPR) have stressed that they were not involved in the attack.
On Friday, the SMM confirmed that Kiev forces and DPR have agreed on the launch of a joint inquiry into the Volnovakha bus attack.