Russia threatened Ukraine yesterday with “irreversible consequences” after a man was killed by a shell fired across the border from Ukraine, an incident Moscow described in warlike terms as aggression that must be met with a response.

Although both sides have reported cross-border shootings in the past, it appears to be the first time Moscow has reported fatalities on its side of the border in the three-month conflict which has killed hundreds of people in Ukraine.

This is total nonsense and the information is untrue

Kiev called the accusation its forces had fired across the border “total nonsense” and suggested the attack could have been the work of rebels trying to provoke Moscow to intervene on their behalf. The rebels denied they were responsible.

Inside Ukraine, combat has intensified dramatically since a rebel missile attack that killed dozens of government troops on Friday. Local officials said yesterday 18 people were killed in shooting incidents in the two main rebel-held cities.

Russia’s Interfax news agency said fierce fighting had broken out on the outskirts of rebel-controlled Luhansk, a city near the border with Russia, and the Ukrainian army had attacked with a force of 70 tanks.

Kiev said it had bombarded a convoy of 100 armoured vehicles and trucks that had crossed into Ukraine carrying in rebel fighters from Russia.

It also said seven of its troops had been killed in attacks. Moscow’s bellicose response to the cross-border shelling 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.

Russia’s Investigative Committee said a shell had 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 one 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, said reports that Ukrainian forces were responsible were “total nonsense and the information is untrue”.

“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.”

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