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

Sweden said Sunday that it seized a vessel suspected of damaging a fiber-optic cable under the Baltic Sea that links the country to Latvia, in what authorities are investigating as an act of "aggravated sabotage.

The Nordic country has opened an investigation into the damage, just weeks after NATO stepped up its military presence in the area following a series of similar incidents.