The group argued that while it was legal, the trade was "immoral" as many nations seek to step up sanctions on the Russian government over its invasion launched in 2022.

Greenpeace members on Saturday filmed the loading of about 10 containers with radioactive labels onto a cargo ship in the Channel port of Dunkirk, the NGO said.

The Panamanian-registered ship, the Mikhail Dudin, is regularly used to carry enriched or natural uranium from France to St Petersburg, according to Greenpeace.

But Saturday's consignment was the first of reprocessed uranium to be observed for three years, it added.

"It is not illegal, but it is immoral," Pauline Boyer, the head of Greenpeace France's nuclear campaign, told AFP.

"France should end its contracts with Rosatom, a state company that has occupied the Ukrainian nuclear power plant at Zaporizhzhia for three years," she added.

French state-controlled energy giant Electricite de France (EDF) signed a 600-million-euro ($700 million) deal in 2018 with a Rosatom subsidiary, Tenex, for the recycling of reprocessed uranium. These operations have not been affected by international sanctions over the Ukraine war.

Rosatom has the only facility in the world -- at Seversk in Siberia -- capable of carrying out key parts of the conversion of reprocessed uranium to enriched reprocessed uranium.

Uranium can be reprocessed so it can be reenriched and reused. With uranium prices rising again on international markets, it is increasingly worthwhile for power companies to seek reprocessing of spent fuel.

Only about 10 percent of the reenriched uranium sent back to France by Russia is used at its Cruas nuclear power plant, in southern France, the only one in the country that can use enriched reprocessed uranium, according to Greenpeace.

France's energy ministry and EDF did not respond to AFP's questions on the consignment or trade.

France ordered the EDF to halt its uranium trade with Rosatom in 2022 when Greenpeace first revealed the contracts in the wake of Russia's invasion.

France said in March 2024 that it was "seriously" looking at the possibility of building its own conversion facility to produce enriched reprocessed uranium.