Shipping firms may need to pay a fee to use the Baltic Sea, one of the world's busiest shipping routes, in order to cover the high costs of protecting undersea cables, Estonia's defence minister said on Wednesday following a spate of breaches.

Nato countries have stepped up patrols to protect critical underwater infrastructure in the Baltic Sea, which is bordered by eight countries, and other waters. A Royal Navy submarine was ordered to surface last November close to a suspected Russian spy ship which was loitering over undersea infrastructure in UK waters.

After a series of suspected undersea cable cuttings, NATO has launched a new surveillance and deterrence mission to protect critical infrastructure under the Baltic Sea.

Sweden has opened a preliminary investigation into suspected aggravated “sabotage” and ordered the detention of a vessel in the Baltic Sea suspected of damaging an underwater fiber optic cable connecting Latvia and the Swedish island of Gotland earlier that day.

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.

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

A Bulgarian shipping company on Monday denied that one of its ships had intentionally damaged an underwater fiber optic cable connecting Latvia and the Swedish island of Gotland.

With its powerful camera, the French Navy surveillance plane scouring the Baltic Sea zoomed in on a cargo ship plowing the waters below — closer, closer and closer still until the camera operator could make out details on the vessel's front deck and smoke pouring from its chimney.

By Johan Ahlander and Anna Ringstrom STOCKHOLM (Reuters) -Swedish authorities boarded a Maltese-flagged ship seized in connection with the latest breach of cables running along the bottom of the Baltic Sea to begin an investigation into the matter,