Denmark detained a container vessel previously blacklisted by Washington under last year's sweeping Iran sanctions on Thursday, amid suspicions it was operating under a false flag.

The Nora was seized after authorities determined it was allegedly sailing under the flag of Comoros without authorization. The ship is now anchored in Danish waters pending further investigation, according to reports. It actually appears to be a box ship transporting containers at the time it was intercepted. It raised the Iranian flag under deeply suspicious circumstances, as a patrol boat eyed the vessel, Danish officials say.

The Danish Maritime Authority believes it to be part of Iran's so-called shadow fleet of tankers. "The Danish Maritime Authority reports that the vessel has been detained due to incorrect registration," the agency said.

Several months ago the vessel went through a name change, which Washington officials believe was in order to keep shipping sanctioned Iranian and Russian exports, and to evade European suspicions while traversing regional waters.

The vessel is said to currently anchored east of Albaek in the northernmost part of Jutland.

It's possible the vessel will eventually be released, as the Danish government explained the ship will be detained until Iran confirms to the agency that the container ship is legitimately registered and certified.

According to more details via a maritime monitoring publication:

Denmark’s TV 2 reports the vessel had gone dark while it was in St. Petersburg, Russia, in mid-January and then sailed west into the Baltic and reached Skagen, where it stopped on January 22. The following day, it anchored less than 20 miles east of Aalbaek, Denmark, where it has remained for the past 28 days.

A Danish patrol ship was spotted near the vessel along with a Danish Armed Forces sea drone. The Danish Maritime Authority reports it questioned the vessel’s registry in Comoros and was informed by the authorities that the ship was “not correctly registered.” Apparently, when they questioned the vessel further, it suddenly raised an Iranian flag, prompting the detention.

Danish outlet TV 2 further reports that the Cerus/Nora had transited Danish waters at least 10 times over the past year during repeated voyages to Saint Petersburg - and each time the vessel allegedly went dark, ceasing transmission of its position data as it neared Russian waters.

The Trump administration is meanwhile contemplating whether to escalate its military pressure on Iran by beginning to directly seize Iranian oil exports. This would be seen by Tehran as an immediate act of war.