Although both sides have reported cross-border shootings in the past, the incident appears to be the first time Moscow has reported fatalities on its side of the border from the three-month conflict which has killed hundreds of people in Ukraine.
Ukraine denied its forces had fired across the border and suggested such an attack could have been the work of rebels trying to provoke Moscow into intervening on their behalf. The rebels denied they were responsible.
The bellicose rhetorical response from Moscow raises the renewed prospect of overt Russian intervention, after weeks in which president Vladimir Putin had appeared intent on disengaging, pulling back tens of thousands of troops he had massed at the frontier.
Russia sent Ukraine a note of protest describing the incident as “an aggressive act by the Ukrainian side against sovereign Russian territory and the citizens of the Russian Federation,” the Russian foreign ministry said in a statement warning of “irreversible consequences”.
“This represents a qualitative escalation of the danger to our citizens, now even on our own territory. Of course this naturally cannot pass without a response,” Russian deputy foreign minister Grigory Karasin told Rossiya-24 state TV.
“What has taken place once again reinforces the need for an urgent end to the bloodshed,” he added, calling for talks between “the sides in the conflict”.
Russia’s Investigative Committee, a crime-fighting body answering directly to Mr Putin, said it had launched a criminal case for murder after a shell landed in the yard of a house in a small town on the Russian side of the frontier, killing a man and wounding a woman.
The Russian town is called Donetsk, sharing the name of the Ukrainian city of 1 million people that the rebels have declared capital of an independent “people’s republic”.
Andriy Lysenko, spokesman for Ukraine’s National Security and Defence Council, called the accusation that Kiev’s forces had fired across the border “total nonsense”, and suggested it might be the work of rebels trying to provoke Russia’s response.
“The forces of the anti-terrorist operation do not fire on the territory of a neighbouring country and they do not fire on residential areas,” he said. “We have many examples of terrorists carrying out provocation shooting, including into Russian territory, and then accusing Ukrainian forces of it.”
The rebels denied blame. Interfax news agency quoted the rebels’ self-proclaimed first deputy prime minister, Andrey Prugin, as saying he was “90 per cent certain” it was Ukrainian troops that had fired across the border.
The conflict in eastern Ukraine began in April when armed pro-Russian fighters seized towns and government buildings, weeks after Russia annexed Ukraine’s Crimea peninsula in response to the overthrow of a pro-Moscow president in Kiev.
The fighting has escalated in recent days after Ukrainian forces pushed the rebels out of their most heavily-fortified bastion, the town of Slaviansk.