WARSAW (Reuters) - Poland is looking into whether any explosive devices were planted where a ship from Russia's "shadow fleet" was seen moving suspiciously near a power cable linking Poland with Sweden on Wednesday, Polish Prime Minister Donald Tusk said.

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

"Shadow fleet" refers to vessels used by Russia to ship oil, arms and grains in violation of international sanctions imposed over its war in Ukraine.

"We are still investigating whether any explosives were planted - it has to be checked very carefully. ORP Heweliusz (Polish hydrographic ship) is still at sea (doing so), and for now there are no worrisome signals," Tusk told reporters on Thursday during a meeting with Polish Navy commanders.

"I am really very satisfied that in a very effective, discreet way, without using unnecessary means, we managed to discourage the ship from any kinetic actions that could cause damage to the power cable," he said.

Defence Minister Wladyslaw Kosiniak-Kamysz said on Wednesday a Polish patrol flight had scared the Russian ship off and the navy's ORP Heweliusz sailed 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.

On Wednesday Russia's embassy in Warsaw declined to comment. In the past, Moscow has denied involvement in undersea sabotage in the Baltic, saying the West was using such claims to curb Russia's seaborne oil exports.

(Reporting by Pawel Florkiewicz and Anna Wlodarczak-Semczuk; editing by Mark Heinrich)