KIEV REUTERS - Ukrainian security officials said on Monday that contact had been lost with an An-26 military transport plane near Luhansk in east Ukraine where there is heavy fighting between Ukrainian forces and separatist rebels.
Also, on Monday Ukraine said its forces had ended a rebel blockade of a strategic airport in the east as it traded charges and threats with Russia over violations of their joint border during a weekend of fierce military combat.
Ukraine's military said its warplanes had inflicted heavy losses on the pro-Russian separatists during air strikes on their positions, including an armoured convoy which Kiev said had crossed the border from Russia.
Ukrainian President Petro Poroshenko's office said Kiev would present documentary proof of incursions from Russia to the international community via diplomats on Monday.
But Russia kept up pressure on Kiev over the death of a Russian man who, it said, was killed by a Ukrainian shell that hit a residential area of a Russian border town.
The Ukrainians have denied the shell was theirs. But a Russian newspaper, citing a source close to the Kremlin, said on Monday that Moscow was considering the possibility of pinpoint strikes on Ukraine in retaliation.
The intensified military activity and Moscow's threat of "irreversible consequences" after the cross-border shelling marks a sharp escalation in the three-month conflict between Ukrainian forces representing Kiev's pro-Western leadership and separatists who have set up 'people's republics' in the east and said they want to join Russia.
Ukrainian forces, taking the lead from Poroshenko who swore to "find and destroy" the separatists who killed 23 servicemen in rocket strikes on Friday, went on the offensive across a broad range of targets south and south-east of the border town of Luhansk and near the town itself.
Poroshenko's office on Monday said Ukrainian forces, backed by warplanes, had broken through rebel lines surrounding Luhansk airport, ending a separatist blockade.
A spokesman for the so-called Luhansk People's Republic said on Monday that 30 volunteer fighters had been killed in Ukrainian fire on Oleksandrivka, a village to the east of the town, Russia's Interfax news agency said.
PUSHING FOR SANCTIONS
Poroshenko on Sunday complained of alleged Russian incursions into Ukraine in a telephone call with the European Union's Herman Van Rompuy with an eye to pushing the 28-member bloc to take further sanctions against Moscow.
The EU - Ukraine's strategic partner with which it signed a landmark political and trade agreement last month - targeted a group of separatist leaders with travel bans and asset freezes on Saturday but avoided fresh sanctions on Russian business.
But a Ukrainian presidential aide said Kiev-based diplomats would be called in on Monday and informed of facts documenting the passage across the border from Russia of military equipment "used in attacks on our serving forces".
"We have the facts and the testimony which we will show to the international community," the aide, Valery Chaly, said, according to Poroshenko's website.
In Moscow, the newspaper Kommersant quoted a source close to the Kremlin as saying pinpoint strikes might be carried out in retaliation for the killing of the Russian man in a border town which bears the same name as Ukraine's main eastern city of Donetsk.
The source said Russia "knew exactly where fire was coming from." He said it would not be a massive action but pinpoint strikes on the positions where the shelling came from.
Russia sent Ukraine a diplomatic note of protest describing the incident as "an aggressive act" against Russia and its citizens and warning of "irreversible consequences".
Andriy Lysenko, spokesman for Ukraine's National Security and Defence Council, denied that Ukrainian forces had fired onto Russian territory and on residential areas. The Ukrainian foreign ministry called on Russian authorities to carry out "an objective and impartial" evaluation of what it described as "a tragic incident".
Moscow's response to the cross-border shelling raises again the prospect of 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.
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 Kiev.
Well over 200 Ukrainian servicemen had been killed in the fighting and several hundred civilians and rebels.
The fighting has escalated sharply in recent days after Ukrainian forces pushed the rebels out of their most heavily fortified bastion, the town of Slaviansk.
Hundreds of rebels, led by a self-proclaimed defence minister from Moscow, have retreated to the Ukrainian city of Donetsk, built reinforcements and pledged to make a stand. The once-bustling city has been emptying in fear of a battle.
Rebel fighters on Monday were evacuating about 200 Donetsk residents by bus across the Russian border into the Rostov area.
Vladimir, a 55-year-old coal miner, was sending his wife with two children to relatives across the border. "The Ukrainians have already cut off water. Electricity is only just working. How can you live without water and light? I have no work but if on top of that I have nowhere to live either there is no reason to be here," he said.