Ukraine's state security service announced Sunday that anti-terrorist units had been placed on high alert due to increased threats against key sites such as airports, stations, pipelines and nuclear plants.

The SBU counterintelligence agency said the measures were "primarily preventative" and made no explicit reference to the mass protests that have been shaking the country since late November.

But it mentioned "threats of explosions" at strategic transport hubs and energy installations as well as "calls to occupy sites sheltering large stocks of weapons" and the "blockading of government buildings".

The protesters who have been occupying central Kiev for more than two months to oust President Viktor Yanukovych have seized several state buildings and repeatedly clashed with police.

The SBU statement said the new measures were "only aimed at ensuring public safety and preventing criminal activities with a terrorist goal".

The agency said "anonymous threats of bomb attacks" had been received but did not go into specifics.

The statement was released after a Ukrainian opposition sympathiser was arrested in Istanbul for attempting to hijack an airliner to Sochi on Friday.

The SBU said the apparently drunk man had demanded that the Boeing 737 be flown to the Olympic resort, where Yanukovych was holding crisis talks with Russian President Vladimir Putin on the sidelines of the Games' opening ceremony.