At a time when the eight NATO nations bordering the Baltic Sea are on high alert after a string of undersea power and internet cables were suspiciously severed, Russia's shadow fleet is getting extra attention.

NATO is ratcheting up its guard against suspected attempts to sabotage underwater energy and data cables and pipelines that crisscross the Baltic Sea.

The drones will be used to monitor the cables and prevent sabotage.

NATO and its eight Baltic Sea allies say they are stepping up efforts to deter 'any attempts at sabotage' in the wake of a series of incidents that have damaged key undersea power and telecommunications cables.

A Bulgarian ship has been released after Swedish authorities cleared it of deliberately sabotaging an underwater cable in the Baltic Sea. NATO has moved to protect Baltic Sea cables amid concerns of Russian tampering.

Estonia's navy is prepared to act against vessels that pose a danger to Baltic Sea infrastructure even if they are in international waters, a senior official patrolling the busy Gulf of Finland shipping lane said on Thursday.

Under steel grey skies, the Dutch minehunting vessel Schiedam cuts through the waters dividing Denmark and Sweden, part of NATO's Baltic Sentry mission to protect underwater infrastructure in the Baltic Sea.