The SPARTA IV, registered as a merchant ship, entered Portuguese waters at the end of the day on Sunday and was sailing off the coast of Nazaré on Wednesday evening.
The Portuguese Navy is monitoring the movements of the Russian-flagged SPARTA IV, which has been sailing "from south to north" along the Portuguese coast since Sunday, Lusa news agency reported, quoting a navy source.
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On Wednesday night, the SPARTA IV was off Nazaré, in the Leiria district. The VesselFinder ship tracking app indicated that the Russian ship left the port of Baltiysk, in the Russian enclave of Kaliningrad, at the end of December.
"The ship is sailing from south to north along the Portuguese coast and is currently off Nazaré, outside the maritime traffic corridors, in areas with depths of more than 3,000 metres," the Portuguese Navy said in a response sent to the Portuguese media.
The SPARTA IV entered national waters on Sunday at 8 pm and has been monitored "by a naval unit" since then, according to the navy. The monitoring will continue until the vessel leaves national waters.
The vessel, described by the Portuguese Navy as a "merchant ship", has transported war material in the past.
In recent years, amid various acts of sabotage targeting Europe's critical underwater infrastructure, the Portuguese Navy has monitored the passage through national waters of more than 100 Russian vessels, including a spy ship belonging to an organisation tied to the Moscow Ministry of Defence.
In 2025 alone, according to navy figures, 69 Russian ships were monitored in Portuguese waters.
In addition, 373 monitoring actions were carried out on ships from Russia's so-called "ghost fleet" as they passed through Portuguese waters, the navy said earlier this year.