By Barbara Erling and Marek Strzelecki

WARSAW (Reuters) - Poland's military intervened after a ship from the Russian "shadow fleet" was seen performing suspicious manoeuvres near a power cable connecting Poland with Sweden, the Polish Prime Minister said on Wednesday.

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.

"A Russian ship from the 'shadow fleet' covered by sanctions performed suspicious maneuvers near the power cable connecting Poland with Sweden," Prime Minister Donald Tusk wrote on X.

"After the effective intervention of our military, the ship sailed to one of the Russian ports."

The term "shadow fleet" refers to vessels used by Russia to ship oil, arms and grains in violation of international sanctions imposed after the invasion.

Speaking later to reporters, Polish Defence Minister Wladyslaw Kosiniak-Kamysz said a patrol flight scared the ship off and Polish Navy Polish Navy's ORP Heweliusz was sailing to the scene.

The Russian embassy in Warsaw did not immediately respond to an emailed request for comment. In the past, Moscow has denied its involvement in undersea sabotage in the Baltic, saying the West was using such claims to curb its sea-borne oil exports.

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. 

A spokesperson for Polish grid operator PSE said the cable was working. PSE data showed over 600 megawatts were flowing to Sweden through the cable at 1130 GMT.

"This shows how dangerous the times we live in are, how serious the situation in the Baltic Sea is," Kosiniak-Kamysz told a news conference. 

"Since Sweden and Finland joined the North Atlantic Alliance, the Baltic Sea has become a key marine area, where the largest number of incidents occur, the most common incidents related to cable breaks... and sabotage."

He vowed a "firm response" from Poland and NATO to any attack on Baltic Sea infrastructure.

Sweden's coast guard declined to comment. Reuters was not immediately able to reach Sweden's minister for civil defence.

(Reporting by Alan Charlish, Pawel Florkiewicz, Marek Strzelecki, Barbara Erling, Terje Solsvik, Stine B Jacobsen; Editing by Alex Richardson and Tomasz Janowski)