KIEV: Six people were treated in hospital for injuries on Tuesday after what police said was a grenade attack on a group of Ukrainian nationalists in the eastern city of Kharkiv which several officials blamed on pro-Russian forces.

The explosion, the latest in a spate of mystery blasts targeting Kharkiv and other Ukrainian cities, occurred on Monday evening as people left a court hearing into a firearms case against a young nationalist.

Kharkiv, a city of 1.4 million, is more than 220 km from the separatist conflict zone further east where Ukrainian government forces are battling pro-Russian rebels.

Right Sector, a far-right group which played a prominent role in street protests that toppled Moscow-backed President Viktor Yanukovich a year ago, said several of its activists had been hurt and pointed the finger at pro-Russian rebels.

“At the end of the court hearing, people began to go out of the courthouse and at that moment there was the explosion. As a result, 13 people were hurt, six of them received wounds,” a police statement said.

Police said the explosion appeared to have been caused by a grenade.

Ukraine’s state security service, the SBU, launched an anti-terrorist operation in the city after Monday’s attack.