Ukraine has accused Russia of shooting down a military plane carrying 20 people in the restive East.
Ukraine's defence minister said that the Antonov-26 transport aircraft was downed by a rocket likely to have been fired from Russia, as it was flying near the border between the two countries.
Earlier, pro-Russian insurgents in eastern Ukraine had claimed responsibility for the downing.
However, defence minister Valeriy Heletey said initial evidence suggested the plane was hit by an advanced-missile system that the rebels do not possess.
Heletey said that, when it was struck, the plane was flying at 6,500m (21,300ft) of altitude, too high to be reached with the shoulder-carried missile launchers used by separatists.
"The aircraft was hit by another more powerful missile, most likely used by the Russian Federation," a statement by the Ukrainian presidency read.
The presidency said the crew had survived and got in touch with the Ukrainian military.
President Petro Poroshenko also reiterated accusations that the Kremlin has been actively supporting rebels with manpower and military equipment.
"In the last three days, Ukraine's armed forces have been attacked with Russian multiple-rocket launchers," Poroshenko said at a meeting of his security officials.
The downing of the Antonov-26 comes a day after Moscow accused Kiev troops of shelling across the border, killing one person and wounding two others ‒ something Ukraine denies.
Nevertheless, the Russia foreign ministry has described the incident as an "act of aggression" warning it could have "irreversible consequences" for Ukraine.
Meanwhile, fighting between troops and separatists intensified around the city of Luhansk.
Ukrainian forces have made substantial gains in recent weeks, taking back large swathes of land from insurgents, who have been forced back into strongholds around Luhansk and Donetsk.