A Ukrainian military transport plane has been shot down along the country's eastern border with Russia but all eight people aboard managed to bail out safely, the defence ministry said.
Separatist rebels in conflict-wracked eastern Ukraine claimed responsibility for downing the Antonov-26, but Ukrainian officials swiftly ruled that out and blamed Russia instead.
There was no immediate comment from Russia on the plane.
In the last two weeks, the government has halved the territory in eastern Ukraine held by pro-Russia separatists, who have been forced back into the cities of Luhansk and Donetsk. Many in the armed insurgency are known to be Russian nationals, but Moscow says they are simply citizens who went to fight in Ukraine on their own.
Defence minister Valeriy Heletey said the plane was flying at an altitude of about 6,500 metres (21,300ft), which he said was too high to be reached with the weapons used by the separatists. Rebels are known to have Igla portable surface-to-air missiles, which work up to about 3,500 metres.
Ukraine's security council spokesman Andrei Lysenko said data from the plane's surviving crew suggested the rocket was either a surface-to-air Pantsir missile or a missile fired by a plane from Russia's Millerovo air force base.
In London, Charles Heyman, a defence analyst who edits a book called Armed Forces Of The European Union, said the missile was more likely fired by the Ukrainian rebels.
"I doubt the transport plane was flying at 6,500 metres. That doesn't make sense. The higher you fly, the more it costs, and the plane would have had to be pressurised," Mr Heyman said. "It was probably shot down using SAM-6 missiles owned by the rebels, which they have quite a few of."
Fighting intensified around Luhansk, meanwhile, as government forces stepped up efforts to disrupt rebel lines and reclaim more territory.
The defence ministry said government troops had retaken several villages around Luhansk - including Metalist, Oleksandrivsk, Bile and Rozkishne - and had reopened a corridor to its civilian airport. Those areas are north, west and south of the city, suggesting the government's plan to form a security cordon around Luhansk is yielding results.
One Luhansk resident named Sergei told the Associated Press that panic had gripped the city over reports that Ukrainian paratroopers were slipping in and detaining rebel fighters. Exit points from the city have been blocked and militiamen were confiscating cars and belongings from residents attempting to flee, he said.
A spokeswoman for the separatist Luhansk People's Republic told the AP that they destroyed a Ukrainian armed convoy in the village of Heorhiivka, killing at least three Ukrainian soldiers.
It was not possible to immediately verify the claim.
Ukraine's authorities insisted again that Russia was directly supporting the separatist insurgency now dragging into its fourth month.
"In the last three days, Ukraine's armed forces have been attacked with Russian multiple-rocket launchers," Ukrainian president Petro Poroshenko said at a meeting with top security officials.
Moscow in turn accuses Ukraine of spreading the unrest to its own territory. Russian media reported yesterday that a Ukrainian shell had hit a building in a Russian border town, killing one person and seriously injuring two others.
Ukraine denied that it had fired shells on to foreign soil but president Vladimir Putin expressed "grave concern" over the incident and Russia's foreign ministry warned there could be "irreversible consequences".
Russian foreign minister Sergei Lavrov sent a letter requesting that observers from the Organisation for Security and Co-operation in Europe visit Russian border towns affected by the fighting. Speaking with Swiss president Didier Burkhalter, chairman of the OSCE, Mr Lavrov called for the resumption of talks to negotiate a ceasefire.
Ukraine's president had a unilateral 10-day ceasefire but abandoned it when rebels would not lay down their arms and return captured border posts.