(Updates with French gov't comment)

By Pascal Rossignol

BOULOGNE-SUR-MER, France, Feb 26 (Reuters) - France on Saturday seized a car cargo ship in the English Channel that Washington says was linked to the son of a former Russian spy chief, in one of the first visible displays of the West enforcing sanctions on Moscow over its Ukraine invasion.

The "Baltic Leader" was headed to St. Petersburg but was diverted to the port of Boulogne-sur-Mer in northern France between 3 a.m. and 4 a.m. (0200-0300 GMT), Captain Veronique Magnin of the French Maritime Prefecture told Reuters.

The vessel was "strongly suspected of being linked to Russian interests targeted by the sanctions," she said.

The United States Treasury Department has issued blocking sanctions against the vessel because, according to Treasury, it was owned by a subsidiary of Russian lender Promsvyazbank, one of the Russian entities hit by U.S. and European Union sanctions.

The bank's CEO, Pyotr Fradkov, is the son of Mikhail Fradkov, a former head of Russia's foreign intelligence service, who also served as prime minister under Russian President Vladimir Putin. Pyotr Fradkov was himself included in the latest round of U.S. sanctions.

Promsvyazbank, in a comment sent to Reuters, said its subsidiary no longer owns the Baltic Leader, and that it was bought by a different entity before the sanctions were imposed.

The French economy ministry, in a statement, said the ship's owner was Promsvyazbank subsidiary PZB Lizing, and that the ship was intercepted in enforcement of EU sanctions on the lender and its subsidiaries.

Magnin, the maritime prefecture spokeswoman, said ongoing checks were being carried out by customs officials and the ship's crew was "being cooperative."

The Russian embassy in Paris will send a note of protest to the French foreign ministry over the seizure, the embassy said in a statement sent to Reuters.

The embassy also said the ship's crew had been allowed to come ashore and move freely about the port.

A Reuters photographer at Boulogne-sur-Mer said the ship was moored at dockside.

The United States, EU, and other Western states this week imposed extensive new financial and trade sanctions on Russia after it sent military forces into neighbouring Ukraine.

The U.S. Treasury Department said Promsvyazbank was put under sanctions "for operating or having operated in the defense and related materiel and financial services sectors of the Russian Federation economy." It was also targeted by EU sanctions. (Additional reporting by Tassilo Hummel and Moscow bureau; Writing by Tassilo Hummel and Christian Lowe; Editing by Frank Jack Daniel)