STOCKHOLM (AP) — Sweden is investigating a sanctioned tanker that is suspected of being part of the sanctioned Russian “shadow fleet,” according to the Scandinavian country’s civil defense minister.

The Swedish Coast Guard said it boarded and detained the Jin Hui on Sunday on suspicion of flying a false flag while transiting Swedish waters.

The vessel was sailing through the Baltic Sea under a Syrian flag, the coast guard said in a statement. There are also concerns about the vessel’s seaworthiness.

The Jin Hui — which remained anchored off Trelleborg on Monday — is currently on European Union, U.K. and Ukraine sanctions lists, Sweden’s Civil Defense Minister Carl-Oskar Bohlin wrote on X.

The ship’s captain, a Chinese citizen, was arrested on suspicion of using a false document and other offenses, Swedish prosecutors said Monday.

It is the fifth vessel seizure by Sweden’s coast guard in recent weeks.

“Ships with suspected deficiencies in their seaworthiness continue to sail in Swedish waters,” said Daniel Stenling, the coast guard’s deputy chief of operations. “This is not acceptable. We have intervened before, now we are intervening again.”

Russia’s ambassador to Sweden, Sergey Belyaev, said in a statement Wednesday that the Jin Hui is not operating under the Russian flag, and said Swedish authorities told the embassy there were no Russians on board.

The statement did not directly address the allegation regarding the shadow fleet.

Sweden last year said it would step up insurance checks on foreign ships, aimed at tightening controls on Russian vessels suspected of transporting oil and gas or carrying stolen Ukrainian grain.