The Jaguar, a tanker the length of nearly five Olympic-size swimming pools, left a port near St. Petersburg, Russia, last year, bound for India and loaded with Russian oil.

Its trip that spring came as Western authorities were frantically trying to piece together the network to which it belonged: one of shadowy ships with hidden owners on whom powerful Russians relied to transport the nation’s valuable oil.

But by a quirk of the shipping industry, the Jaguar had ties to the West. The tanker flew the flag of St. Kitts and Nevis, which has its maritime registry just outside London — some 20 miles from the very British authorities who chase Russia’s assets around the world and chart its oil shipments.

After unloading the oil, the Jaguar would soon switch to a more obscure flag, the Central African nation of Gabon. With an act of paperwork, the Russian tanker had moved beyond the reach of Western financial authorities.

Dozens of tankers have made similar moves over the past year and a half, records show, as Moscow has worked to protect its so-called dark fleet in the face of international pressure to limit the market for Russian oil.

It is the latest in a cat-and-mouse game that President Vladimir V. Putin of Russia has played with the West since Moscow’s full-scale invasion of Ukraine in February 2022. As American, European and British authorities have pursued, frozen and seized Russian money worldwide, Mr. Putin has found ever new ways to evade their grip.