A tanker carrying Russian oil that was secured by German authorities last week in the Baltic Sea remains in German waters and a customs assessment is underway, a government spokesperson said in Berlin on Wednesday.

"German customs authorities are currently checking whether all the conditions and requirements that need to be met are being fulfilled," the spokesperson told reporters, adding that the vessel would remain in German waters until the process is complete.

The 274-metre (899-foot) Panama-flagged tanker, called Eventin, was stopped while adrift and unable to manoeuvre on Friday with 99,000 metric tons of oil on board, according to Germany's Central Command for Maritime Emergencies.