As they plied the gray, icy waters of the Baltic Sea west of Russia on Thursday, the crew of the Estonian minehunter EML Sakala kept a careful eye on any vessels slowing down suspiciously or suddenly changing course.

Officials say the damage to Estlink 2, a power connection with the capacity to provide half of Estonia's winter electricity needs, did not disrupt service, although it did drive up energy prices in the country and the other Baltic nations.

Incidents of suspected sabotage of undersea cables and pipelines are prompting countries to step up security measures. Estonia's navy has launched an operation to try to counter sabotage, after cables were damaged in December.

NATO's standing maritime group is to set off from its base in the German port of Kiel for the Baltic Sea to monitor critical underwater infrastructure following several suspected acts of sabotage, the group's commander said on Friday.

NATO is increasing its surveillance operations in the Baltic Sea region with two ships deployed to the area following suspected sabotage of undersea cables between Finland and Estonia.

If the Russians see that we are present there, the likelihood of such sabotage acts immediately decreases," said one navy commander.