The ship Tagor was reportedly stopped in international waters on Sunday morning. According to prosecutors in Brest, France, the ship's captain refused to follow the navy's orders, and a criminal investigation has been launched.
Russia is furious. The Kremlin calls the seizure illegal and equates it with "piracy."
"Russia is taking measures to secure the cargo," spokesman Dmitry Peskov said.
According to ship-tracking services, the Tagor was off the coast of Norway last week.
The EU's list of sanctioned Russian "shadow vessels" - ships that are considered to be used to circumvent sanctions against Russian oil exports - amounts to almost 600.
