A Ukrainian military transport plane was shot down in the east of the country on Monday.
It was carrying eight people and the Ukrainian Defence Minister said a search-and-rescue operation was underway to find them.
According to Russian media reports, rebel forces say they have captured the crew and are questioning them.
President Petro Poroshenko held an emergency meeting with his security chiefs.
Ukrainian officials said the missile that downed the plane was a powerful one and probably fired from Russia.
“It has become clear that the plane was shot down at the height of 6,500 meters. Mobile rocket launchers, which are used by rebels now, cannot shoot the plane down at this height, which means AN-26 was shot down by another more powerful weapon which was used probably from the territory of the Russian Federation,” said Ukrainian national security and defence spokesman Andriy Lysenko.
Nato has reported a build-up of Russian troops near the border.
Russia denies supporting and arming the separatists, and has invited officials from the OSCE to monitor its border with Ukraine.