By Janis Laizans

GULF OF FINLAND (Reuters) - 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.

The Baltic Sea region is on alert and the NATO alliance has boosted its presence after a series of power cable, telecom and gas pipeline outages since Russia invaded Ukraine in 2022. Most were caused by civilian ships dragging their anchors.

While some of the underwater breaches have been ruled accidental, others are still under investigation although no suspects have so far been put on trial.

On a cold morning with snowy weather limiting visibility, the mine hunter the Sakala approached a barge, which had a Cypriot flag but appeared to have a Russian name. From a distance of a few hundred metres, it used a camera to zoom in on the vessel's anchors.

On this occasion, nothing wrong was detected, and the navy ship continued patrolling the area through which around 500-600 vessels pass every week, many heading to or from Russian ports.

"If there is a threat that they are going to break the critical infrastructure, then we are forced to intervene," Estonian navy Deputy Commander Johan-Elias Seljamaa said, adding that this would be the case "even in the international waters".

He declined to discuss operational details.

The patrols began after Finland in December seized an oil tanker suspected of ripping apart the Estlink 2 undersea power cable by dragging its anchor along the seabed, cutting one of the two electricity lines that connect the country with Estonia.

To try to deter any future saboteurs, Estonia has since deployed three vessels near the remaining Estlink 1 cable in the busy Gulf of Finland, an arm of the Baltic Sea that at its eastern end stretches into Russian waters.

Moscow has denied any involvement in the cable breaches.

(Reporting by Janis Laizans, writing by Terje Solsvik; editing by Barbara Lewis)