A Ukrainian transport aircraft was "probably" shot down from within Russian territory, Ukraine's Defence Ministry has claimed. Pro-Russian rebels had previously claimed they had shot down the AN-26 transporter.
Ukrainian officials lost contact with the aircraft's crew near rebel-held Luhansk at lunchtime.
The Defence Ministry said some crew of the downed plane had been in contact with the army's general staff.
A statement on the Ukrainian Presidency website said only missiles "likely from the territory of the Russian Federation" would have been able to reach the aircraft at the altitude it was flying.
It was not immediately clear if there were any casualties on the aircraft, which officials said could have been carrying about 20 people.
Fighting has been intensifying around Luhansk as government forces stepped up efforts to disrupt rebel lines. Warplanes inflicted heavy losses on the separatists during airstrikes overnight, Ukraine's military said.
Government troops retook several villages around Luhansk and reopened a corridor to the airport, it was claimed.
Russia 'sending officers to fight with rebels'
Meanwhile, Ukraine's President Petro Poroshenko has accused the Russian military of sending officers to fight alongside the rebels in the east of the country.
He also claimed that a newly developed Russian missile system had been used against Ukrainian forces during the previous three days.
Amid the heightened tensions, Russia said it had invited the OSCE security monitoring group to two of its border crossings with Ukraine as a sign of "goodwill".
At the weekend, Moscow claimed a man in Russia had been killed by a shell that had come from across the border.
But Ukraine's National and Security Council spokesman Andriy Lysenko accused rebel fighters of being behind the cross-border shelling.
He told journalists on Monday morning "The (rebel) fighters systematically fire mortar and shoot into Russian territory which killed a Russian citizen".
On Sunday, Vladimir Putin met with Angela Merkel at the World Cup in Brazil. The German Chancellor urged the Russian President to use his influence over the separatists to help bring about an end to fighting.
In the last two weeks, the government has halved the amount of territory held by the pro-Russia fighters.