Up to 850 oil tankers are estimated to form the so-called shadow fleet transporting oil from countries such as Iran and Venezuela as well as Russia, which has multiple restrictions on its oil exports.

V.Group's CEO Rene Kofod-Olsen said they "lost 30 ships to the dark fleet" last year.

"Typically we will try and retain management of a new owner. Clearly that was not possible for us," he said on the sidelines of Posidonia shipping week in Athens, with new owners failing to pass the company's compliance standards.

Maritime services provider V.Group works with over 3,500 ships with close to 300 tankers under its management through subsidiary V.Ships, and has one of the largest crewing pools of seafarers in the world.

Kofod-Olsen said since last year they had been approached by other new companies seeking V.Group to manage their vessels, but those also did not meet compliance checks.

Ships carrying sanctioned oil cargoes pose a massive environmental challenge, industry and analyst sources say, since they are hard to track because of their opaque ownership and use of non-Western insurance and other marine services, and have little incentive to follow cleaner shipping standards.

Kofod-Olsen said the wave of shadow fleet tanker acquisitions had abated lately. "Ship owners that we know are much more conservative in terms of trading assets," he said.

Nevertheless, the existing shadow fleet was active.

"These assets are ageing, the risk is just going to go up," he said. "They are sailing full of fossil fuels or molecules and they are a sailing danger."

Others also raised the alarm in Athens this week.

"If elements within the industry circumvent the rules and regulations, we have a problem," said Harry Conway, chair of UN agency the International Maritime Organization's Marine Environment Protection Committee.

"Dark fleet vessels have no accountability because they operate under the radar - they don’t respect the rules," Conway told a Tradewinds Shipowners Forum at Posidonia.