Six people were treated in a 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 kilometers 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 Yanukovych 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, which officials said had clearly targeted people, unlike previous blasts in Kharkiv, the southern city of Odessa and elsewhere.
"We see a link with this and preceding explosions in Kharkiv and Odessa. As to who carried this out, it is our home-grown traitors, the pro-Russian elements," said Markiyan Lubkivsky, a senior SBU official.
"We have no doubt this was a well-organized system which is directed from across the border from Russia."
Kharkiv is a major defense center producing high-tech military equipment for Ukraine's defense industry. Other sites targeted in the city in recent months include a National Guard base, a military hospital and a bar where money was being collected for Ukrainian soldiers.
Andriy Sanin, a local Right Sector activist, said: "It is clear this was an attack on the lads of Right Sector. The pro-Russian terrorists have already several times carried out terrorist attacks in our city. This is their work."
"The pro-Russian bandits will pay for this in full," a Right Sector statement said.