The risk of a large Baltic Sea oil spill has risen since Russia began using a "shadow fleet" of tankers to circumvent curbs on its oil exports, Finland's Border Guard warned Thursday.

Experts say the so-called ghost fleet of tankers with opaque ownership or without proper insurance has allowed the Kremlin to keep exporting despite a Western-imposed embargo and oil price cap on its global sales.

"Certainly our assessment, especially during the last year, has been that there is an increased risk of an accident with environmental consequences," the head of the Finnish Border Guard Headquarters maritime safety unit, Mikko Simola, told AFP.

He explained that around 70 old "ghost" oil tankers, which can each carry more than 100,000 tonnes of crude oil, navigate through the Gulf of Finland every week.

The number of vessels navigating the area "is approximately the same or even higher than before the Russian attack on Ukraine," Simola said.

All oil tankers departing from the big Russian harbours of Primorsk, Ust-Luga, Vysotsk, and Saint Petersburg use the same route through the Gulf of Finland, bordering Finland and Estonia.

The old tankers' poor condition increases the risk of technical failures that could lead to a dire oil spill, particularly with the Baltic Sea's shallow waters and fragile ecosystems.

"Another worrying factor is the uncertainties in ownership and insurance coverage of the ships," Simola added, citing flag states that "had not been seen before in the northern Baltic Sea" such as Cameroon, Gabon, Djibouti, Palau, and Belize.

These states are presumably not familiar with navigating in Nordic ice conditions, which adds to the risk of an environmental catastrophe, he said.

The Finnish Border Guard intensified its monitoring of the sea shortly after Russia's full-scale invasion and the imposition of Western sanctions.

"We are doing our utmost to be constantly prepared for marine environmental response activities, would there be a need," Simola said.

In the event of a major oil spill in the area around the Gulf of Finland, coast guards from neighbouring countries Estonia and Sweden could also be called upon for assistance.