The diversion of a Ryanair flight by Belarus last week was an act of "state piracy", EU commissioner Thierry Breton told AFP on Monday after meeting some of the passengers in Lithuania.

"It is an unacceptable event. There has been a hijack, not just of an EU plane with European passengers, but a hijack of European values and that we will not tolerate," he said.

The Athens to Vilnius flight on May 23 was forced to land in Minsk and dissident journalist Roman Protasevich and his girlfriend Sofia Sapega were arrested, causing a global outcry.

Europe "has been under attack with this act of state piracy. We will not leave this unpunished," Breton said, adding that the EU was working on additional economic sanctions on Belarus.

Exiled Belarusian opposition leader Svetlana Tikhanovskaya during a visit to Estonia on Monday discussed possible EU economic sanctions with Estonian President Kersti Kaljulaid.

Belarusian President Alexander "Lukashenko has brought Belarus on a path to self-destruction for the sake of personal power and glory," she said at a joint press conference with Kaljulaid.

"His regime is now a threat to regional and European security," she said.

Tikhanovskaya and Kaljulaid were both dressed in white and there were white and red flowers on display -- the colours of the former Belarusian flag which is now used by the opposition.