At least five tankers from Russia’s so-called “shadow fleet” — vessels used to evade Western sanctions — have leaked oil into European waters over the past year, an investigation by Politico and the journalist group SourceMaterial has said.

Citing satellite imagery from the nonprofit SkyTruth and shipping data from the analytics platform Kpler, the investigation showed that the vessels continued to operate largely unchecked after leaving oil slicks near Europe’s coastline.

Two of the ships were already under British sanctions when the leaks occurred.

Latvian Energy Minister Kaspars Melnis called the incidents “a huge problem,” warning that Europe is “quite lucky at this moment that we don’t have any environmental catastrophe happening.”

A British Foreign Office spokesperson described the shadow fleet as “a desperate and dangerous attempt” by Putin “to cling onto his oil profits and polluting the sea in the meantime.”

“He’s using ships that ignore basic safety standards, increasing the chance of catastrophic oil spills,” the spokesperson told Politico.