Russia threatened Ukraine on Sunday 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 that has killed hundreds of people in Ukraine.
Russia has made repeated claims that settlements along its porous border with Ukraine - which the West and Kyiv say is a key supply route for the rebels - have been hit by Ukrainian fire, but no deaths have been previously reported.
Kyiv 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.
Fighting near Luhansk
Meanwhile, Russian and Ukrainian news reports say Ukraine forces attacked the perimeter of the rebel-controlled city of Luhansk Sunday with armored columns, pushing into the outskirts of the city but failing to recapture it.
Few details were reported. But rebels and residents near the Russian border were quoted as saying the attacks occurred just south and west of the city.
A correspondent for Russia's Rossiya 24 state television said 90 Ukraine tanks and armored personnel carriers spearheaded the attacks, and that rebels countered with tanks and other military hardware seized in previous fighting.
Residents and pro-Russian separatists said the city of of 425,000 remained under rebel control late Sunday.
Talks between Russia and Ukraine over a cease-fire between the rebels and Kyiv's troops have stalled in recent weeks, as Ukrainian troops have succeeded in pushing insurgents out of key towns in the east.
Moscow's harsh response to the cross-border shelling raises the renewed prospect of overt Russian intervention, after weeks in which Putin had appeared intent on disengaging.
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 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 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 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, because the rebels were short on ammunition and cautious about where they fired.
The conflict in eastern Ukraine erupted 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 Kyiv.
Civilians on the move
The fighting has escalated sharply in recent days after Ukrainian forces pushed the rebels out of their most heavily-fortified bastion, the town of Slovyansk.
Ukraine's Donetsk, where rebels have gathered to regroup after a major Ukrainian offensive last week, was quiet on Sunday, many residents leaving for fear of increased fighting.
About 150 people from the settlement of Marynka, on the outskirts of the city, were moving into dormitories at a local university on Sunday, after their homes were bombarded during the night.
“Everybody here is sitting on a suitcase. People are only prevented from leaving by work - that is if they have any work. If they (the Ukrainian forces) are going to bomb, then I shall, of course, go, too,” said Olga, 35.
On the streets there are fewer and fewer cars. Some drivers no longer bother to stop at red lights since there are no police around and few vehicles.
Airstrikes on rebels
Ukrainian defense officials said Sunday that the air force had performed 16 sorties and carried out five airstrikes on rebel positions over the previous day.
Kyiv says Moscow has provoked the rebellion and allowed fighters and heavy weapons to cross the border with impunity. It has struggled to reassert control over the eastern frontier, recapturing border positions from rebels.
The conflict in Ukraine, which has claimed more than 550 lives and enflamed East-West ties, threatened to spiral into an all-out civil war over the weekend.
Militias that the West and Kyiv allege are being armed by the Kremlin used a Grad multiple-rocket system late Friday to mow down 19 Ukrainian soldiers and wound nearly 100 near the Russian border.
Further attacks killed 18 more troops and 20 civilians - 12 of them in what Kyiv said were missile and other overnight rebel strikes staged across the eastern rustbelt - in violence that appeared to shatter any hope of a truce.
The separatists' use of Grad systems - featured heavily in Russia's devastating assault on the Chechen capital Grozny in the 1990s - has underpinned the most recent charges that the Kremlin is directly involved in the insurgency.
Poroshenko has vowed to kill "hundreds" of gunmen for every lost soldier and ordered an air-tight military blockade of Luhansk and Donetsk - both self-proclaimed capitals of their own "People's Republics" that want to join Russia.
Kyiv said it killed hundreds of rebels in air strikes on Saturday, although there was no independent confirmation of such high casualties and the rebels denied suffering serious losses.
Armored convoy attacked
Ukrainian security spokesman Lysenko said on Sunday forces had used artillery to strike a convoy of about 100 armored vehicles and trucks after confirming that the convoy was carrying “a large number of recruits” into Ukraine from Russia.
He said seven Ukrainian service members had died in attacks in the east in the past day.
The Donetsk city council said in a statement on its website on Sunday that 12 people had been killed at a mining settlement near the Ukrainian city. It gave no details of who had fired.
Municipal authorities in Luhansk, capital of the other rebellious eastern province, said six people were killed in clashes there. It also gave no details of who was to blame.
Western countries have threatened to impose harsh economic sanctions on Moscow if it intervenes openly. Russia denies fueling the conflict, but Kyiv and Western countries say it has supported the rebels.
Some information for this report provided by Reuters, AP and AFP.