(Bloomberg) -- The number of ships sailing under the flag of Gabon is surging as the US steps up scrutiny on other nations associated with so-called dark-fleet vessels, according to maritime intelligence firm Windward AI.

The number of vessels that were switched to the African nation’s flag jumped fivefold last month, compared with the monthly average over 2023, according to Windward data. That means there were more than 100 dark-fleet vessels flagged to Gabon at the end of January, up from just 20 in February 2023.

“The Gabon flag registry started to gain popularity as a flag of convenience, offering stakeholders an easy and cheap haven,” Ami Daniel, chief executive officer and co-founder of Windward AI, said in an interview. It’s a move aimed at staying away from the focus of regulators, he added.

The US and allies have been ramping up enforcement of a price cap on Russian oil exports in recent months, targeting traders and firms that help haul crude above the threshold as part of efforts to push back against Moscow’s invasion of Ukraine. Regulators have also been increasing scrutiny of several nations whose ship registries have commonly associated with illicit activity, said Daniel.

“Flags like Liberia, St. Kitts and Nevis and the like have been heavily targeted by regulators - making them less attractive to bad actors,” he said. In the fourth-quarter of 2023, the top three flag-states for dark-fleet vessels were Panama, Liberia and Russia, according to a Windward AI report.

Some 50 of the vessels reflagged to Gabon in the last month switched from the Liberia and Panama flags, and were owned by companies based in Russia, according to Windward AI. All ships must register with a flag state, which then enforces international maritime regulations under UN laws.

The so-called dark fleet — a group of vessels characterized by opaque ownership and use of deceptive practices such as disabling tracking systems - has expanded rapidly since Russia’s invasion of Ukraine in 2022. US restrictions on Russian and Iranian oil, as well as previous ones on Venezuelan cargoes, have created a booming trade for these old vessels operating outside of western oversight.

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