DONETSK, Ukraine -- Russia threatened Ukraine on Sunday with "irreversible consequences" after a Russian man was killed by a shell fired across the border, while Kiev said Ukrainian warplanes struck again at separatist positions in the east of the country, inflicting big losses.
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 that has killed hundreds of people in Ukraine.
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.
Ukrainian President Petro Poroshenko, who earlier turned down an invitation to attend the World Cup soccer final in Brazil where he might have met Russian President Vladimir Putin, accused Russian forces of crossing the border and attacking Ukrainian servicemen.
In a telephone conversation with the European Union's Herman Van Rompuy, he called on the EU to consider "the illegal crossing of the Russian-Ukrainian border of heavy military equipment and an attack by Russian soldiers on the positions of Ukrainian servicemen," his website said.
His comments were linked to an earlier report Sunday by the government's "anti-terrorist operation" that a convoy of about 100 separatist armored vehicles and trucks had crossed into Ukraine carrying rebel fighters from Russia. A Ukrainian military spokesman said Ukrainian artillery had destroyed the column.
Combat has intensified dramatically in Ukraine since a rebel missile attack killed dozens of government troops Friday.
Ukrainian forces said their warplanes Sunday carried out five air strikes on rebel bases near Luhansk airport, at Izvarino on the border and against Chechen fighters who it said had occupied a National Guard base at Lysychansk.
Rebels controlling Luhansk, on the border with Russia, said Ukrainian forces had now begun to storm the town with about 50 tanks and attack planes. There was no word of this from the Ukrainian side.
In other incidents, officials said 18 civilians had been killed in shooting in Luhansk and Donetsk, the region's main city, which is also controlled by separatists.
Moscow's bellicose response to the cross-border shelling raises again the prospect of Russian intervention, after weeks in which Mr. 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."
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.