Ukraine accused Russia of sponsoring a terrorist attack on a peace march in the city of Kharkiv that left two dead and at least 10 wounded.
In the deadliest blast of a bombing campaign by pro-Russian separatists, an explosive device was thrown from a car towards marchers who were commemorating the deaths of more than 100 protesters in last year's revolution.
Ukrainian authorities said they had arrested four suspects shortly after the attack, claiming that the men had received training in a Russian city just across the border.
"They are Ukrainian citizens, who underwent instruction and received weapons in the Russian Federation, in Belgorod," said Markian Lubkivskyi, an aide to the head of the Ukrainian security agency, the SBU.
The blast was the latest in several months of bomb attacks on targets in government-held cities including Kharkiv, Odessa, Mariupol and Kiev. Previous bombings tended to avoid casualties, targeting the offices of pro-Kiev groups or infrastructure at night. Before yesterday, the bloodiest attack had been the bombing of a bar in Kharkiv on November 9 that injured 11 people.
Kharkiv is 225km from the conflict zone but is considered by pro-Russian separatists to be part of Novorossia, a historic term for lands annexed by Moscow in the 18th century on which the rebels base their territorial ambitions. It was the scene of violent pro-Russian protests in spring last year, but an attempt to set up a breakaway "People's Republic" like those established in Luhansk and Donetsk lasted just hours before being put down by police. It is now firmly under government control and its many arms factories play a key role in the Ukrainian war effort.
Earlier in the day Russian-backed separatists said that they would pull back heavy weapons in accordance with the Minsk peace accords, raising hopes that a truce could be salvaged despite the failure of a week-old ceasefire.