HELSINKI (Reuters) - The Russian navy twice interfered with a Finnish state environmental research vessel in international waters in August and September, the Finnish Environment Institute said on Saturday.

The statement comes after Finland accused Russia, its eastern neighbor and former ruler, of violating its airspace three times in less than a week in August.

Finland's foreign ministry said it was concerned by the preliminary information it had received about the incidents and that Finnish defense forces were investigating.

The institute said its research vessel, which is used for monitoring the Baltic Sea, was heading to take samples from a routine location near Sweden in August when a Russian naval vessel told it twice via radio to change course.

It obeyed the first but ignored the second request, and later noticed a submarine nearby.

"In September, a Russian helicopter and a military ship passed the vessel very close," said Mari Walls, the institute's marine research director.

The crew and Swedish researchers onboard considered the incident as threatening, she said. Walls declined to speculate on the Russian navy's motives.

The Russian defense ministry said it could not immediately comment.

European Union member Finland shares a 1,300-km (800-mile) border with Russia and maintains generally cordial relations with Moscow. However, Russia's involvement in the conflict in Ukraine has led Finland's defense forces to step up surveillance.

(Reporting by Jussi Rosendahl in Helsinki; Additional reporting by Alexander Winning in Moscow; Editing by Raissa Kasolowsky)