(QHA) -

Human Rights Watch (HRW) says pro-Russian separatists are "probably responsible” for two recent rocket attacks that killed more than 40 civilians in eastern Ukraine.

In a February 3 statement, the New York-based group said its "on-the-ground investigation" indicated rebel forces fired unguided rockets that killed at least 12 civilians on a bus in the town of Volnovakha on January 13 and at a least 30 civilians in the Azov Sea coastal city of Mariupol on January 24.

HRW also says circumstances point to government forces’ responsibility for an unguided rocket attack in Donetsk that killed two civilians there on January 13.

It said it was unable to attribute responsibility for a January 22 attack in Donetsk that reportedly killed 13 civilians.

Fighting in the conflict that has killed more than 5,100 people since April intensified around January 10, after a relatively calm two-month period. 

HRW said both sides have used "explosive weapons with wide-area effect in populated areas."