According to the ship tracking site Marine traffic, Flora 1 departed the Russian oil port Primorsk headed for Santos in Brazil, flying the flag of Sierra Leone.
Sweden's coastguard said it boarded a vessel subjected to EU sanctions on Friday, suspecting it of an environmental crime after an oil spill in the Baltic Sea.
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The Flora 1 tanker vessel was boarded after an oil spill stretching 12 kilometres was detected early on Thursday east of the Swedish island of Gotland, the coastguard said in a statement.
The vessel was escorted to an anchorage near Ystad in southern Sweden.
"Upon discovery, it was determined that the vessel is on the EU sanctions list and several unclear issues surrounding the vessel were identified, including its flag status," the coastguard said.
The vessel had "an unknown flag status" and was "en route from a port in the Gulf of Finland with an unknown destination," it said.
It was loaded with oil and had 24 crew members on board.
Moscow's "shadow fleet" consists of vessels used to skirt Western sanctions. They are often ageing ships in poor condition, of opaque ownership and without proper insurance.
"The government takes this incident seriously, even though it does not involve a major oil spill this time," Civil Defence Minister Carl-Oskar Bohlin said on X.
The Russian shadow fleet, he said, "poses a significant safety and environmental threat."
"A more extensive spill could have had devastating consequences for marine ecosystems and the Swedish coastline," he added.
A preliminary investigation into a suspected environmental crime had been launched and an investigation was being conducted on board the vessel.
According to the ship tracking site Marine traffic, Flora 1 departed the Russian oil port Primorsk headed for Santos in Brazil, flying the flag of Sierra Leone.
Sweden's coastguard has already carried out previous boardings of suspect vessels.
On 12 March, the coastguard boarded the Sea Owl I in territorial waters and opened a false flag investigation.
Less than a week earlier, it had intercepted another suspected false-flagged cargo vessel in the same area.