Ukrainian security forces have found evidence that independence-seeking insurgents in the east were "plotting a terror act" against a plane of Russia's Aeroflot airline, a top security official said Thursday.

"During the investigation of a terror attack against Malaysian Boeing, Ukrainian law enforcement and intelligence agencies found that on July 17, on day of Boeing crash, the military mercenaries were planning to carry out a terrorist attack against the Aeroflot aircraft," Valentin Nalivaichenko, the head of the Ukrainian Security Service, told reporters.

Nalivaichenko said the terror attack was planned against a plane en route from Moscow to Lanarca, Cyprus, whose course was close to the flight path of the crashed Malaysia Airlines Flight MH17.

He did not specify the evidence, but said the attack on the Russian plane was aimed at "creating a pretext for Russia's invasion in Ukraine."

MH17, a Boeing 777 jetliner en route from Amsterdam to Kuala Lumpur, crashed in eastern Ukraine on July 17, killing all 298 people on board.

The crash was shrouded in mystery amid suspicions that it was shot down by a missile. The Ukrainian government and insurgents in the country's east have traded barbs, accusing each other of being responsible for the tragedy.