Belgium said it is investigating the presence of a Russian "spy ship" in the North Sea, a day after its neighbour the Netherlands reported that Russia was targeting North Sea infrastructure.

The ship was detected in the Belgian North Sea around mid-November last year, Vincent Van Quickenborne, the Justice and North Sea minister, said in a statement headlined "Russian spy ship off our coast in November".

While the presence of Russian ships in the North Sea is not forbidden, Belgium said it was monitoring the situation closely, in the context of the war in Ukraine.

"We don't know the exact motives of this Russian ship, but let's not be naive," Van Quickenborne said. "Especially if it behaves suspiciously close to our wind farms, undersea gas and data cables and other critical infrastructure."

The Dutch military intelligence agency said on a Monday it detected a Russian ship at an offshore wind farm in the North Sea as it tried to map out energy infrastructure. The Dutch navy and coast guard escorted the ship out of the area.

Since the start of the year, the Belgian Maritime Security Act has come into force and allows, among other, for camera surveillance at sea, with mobile cameras on drones or ships. The government says it will use this to monitor wind turbines and will take necessary measures.

Moscow has not commented on the Dutch and Belgian reports about the ship. (Reporting by Marine Strauss,)