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 1 million people that the rebels have declared capital of an independent "people's republic.”

Andriy Lysenko, spokesman for Ukraine's National Security and Defense 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 neighboring 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 Ukrainian foreign ministry echoed this view and called on Russian authorities to carry out "an objective and impartial" evaluation of what it described as "a tragic incident". It said Ukrainian authorities would cooperate in any investigation.

The rebels denied blame. Interfax news agency quoted the rebels' self-proclaimed first deputy prime minister, Andrey Prugin, as saying he was "90 percent certain" it was Ukrainian troops that had fired across the border. (rt/ez)