WARSAW (Reuters) -A Russian ship from the "shadow fleet" covered by sanctions performed suspicious manoeuvres near a power cable connecting Poland with Sweden, Polish Prime Minister Donald Tusk said in a post on X on Wednesday.

"After the effective intervention of our military, the ship sailed to one of the Russian ports," he added. He said the Polish Navy's ORP Heweliusz was sailing to the scene.

The 600-megawatt undersea cable links the Swedish coast near Karlshamn with Ustka in northern Poland and allows both grids to rely on cross-border supplies when electricity is cheaper in the other system.

NATO has stepped up security in the Baltic following a string of incidents in which power cables, telecom links and gas pipelines have been damaged in the wake of Russia's invasion of Ukraine in February 2022.

Polish Defence Minister Wladyslaw Kosiniak-Kamysz said the cable belonged to Polish grid operator PSE.

(Reporting by Alan Charlish, Pawel Florkiewicz, Marek Strzelecki, Barbara Erling; Editing by Alex Richardson)