A Malaysia Airlines Boeing 777 with more than 280 passengers on board crashed in eastern Ukraine on Thursday, Russian and Ukrainian news agencies report.
The airline said on its Twitter account that it lost contact with flight MH17 from Amsterdam to Kuala Lumpur. "The last known position was over Ukrainian airspace," it said.
The fate of the passengers remains unclear.
Both Russian and Ukrainian sources confirmed that the plane went down between the city of Donetsk and the Russian border, an area that has seen heavy fighting between pro-Russian separatists and Ukrainian government forces.
Ukrainian Interior Ministry official Anton Gerashchenko said that the plane was downed with a surface-to-air missile system supplied to the rebels by Russia.
Separatist leaders have denied the accusation and suggested that the plane was shot down by Ukrainian forces.
Andrei Purgin, a leader of the self-proclaimed Donetsk People's Republic, told Interfax that the insurgents had no Russian-made "Buk" missile systems.