At the shipyard in Saint Nazaire the superstructure sticks out like a sore thumb, a great tower surrounded by cranes and lifting devices that signal to the town that the helicopter carrier is moored here, in the oily waters of the naval dockyard. Peering through the barbed wire, you can pick out its name, Vladivostok, in Cyrillic script on its hull. A Russian Orthodox priest came to baptise this 200-metre monster. Further along, in a dry dock, its younger sister, Sebastopol, has also been promised to the Kremlin. Two “grey” ships, as they call naval vessels at the yard; two stumbling blocks for European diplomacy as it attempts to toughen sanctions against Vladimir Putin, just as the Vladivostok starts its sea trials.

These two vessels are technically Mistral class or bâtiment de projection et de commandement, projection and command ships. For the average person, they are assault ships, capable of transporting and landing troops, armoured vehicles and tanks. Carrying helicopters, they can serve as a base for airborne commando units. In the French navy they represent the largest tonnage after the Charles de Gaulle aircraft carrier. If the Russian Black Sea Fleet had possessed a Mistral-class warship in 2008, it could have finished its war on Georgia in “40 minutes instead of 26 hours”, the head of the Russian navy, Admiral Vladimir Vysotsky, once boasted.

The contract to built the ships was signed by President Nicolas Sarkozy in 2011, long before Putin showed any signs of attacking Ukraine, annexing Crimea or encouraging secession by the predominantly Russian-speaking self-styled republics of Donetsk and Luhansk, well before a ground-to-air missile brought down a Malaysia Airlines plane in July. But Hollande has no wish to go back on a contract worth €1.2bn ($1.5bn). At the beginning of September, on the eve of the Nato summit in Wales, Hollande announced France could not go ahead with the Vladivostok’s delivery to Russia, citing Moscow’s actions in eastern Ukraine. However the partial ceasefire in mid-September meant the French permitted the ship to begin its sea trials.

At the Nato headquarters in Brussels, member states are flabbergasted that France should be selling warships to a country that is threatening their security. In Washington Barack Obama is furious too.

Only in Saint Nazaire, Brittany, do they seem happy about the presence of the “Sebass” and “Vladi”, nicknames that reflect the locals’ attachment to their cumbersome guests. Russian sailors arrived at the end of June. They boarded the Smolny, their training ship, at Kronstadt, and it remains moored near the lock gates. Prefabricated huts on the quayside serve as classrooms for the cadets. Nets have been strung along the port side of the Smolny, to stop divers coming too close to the old ship, built in Szczecin, Poland, in 1976. “That thing wouldn’t be seaworthy in a gale,” says a naval veteran on the port.

A demonstrator at the STX dockyards in St Nazaire against the delivery of the controversial French-built helicopter gunship Vladivostok ordered by Russia. Placard reads: ‘No testing at sea for Putin’s killers.’ Photograph: Jean-Sebastien Evrard/Getty

In some odd way the whole line-up is a reminder of revolutionary history. In 1921 sailors at the Baltic port of Kronstadt mutinied, only to be put down by the newly established Bolshevik regime. Szczecin, with its shipyards, was a Solidarnosc stronghold in the struggle against Soviet domination. Smolny, as the trade union faithful on the port at Saint Nazaire still recall, was “Lenin’s headquarters, from which he directed the attack on the Winter Palace”, according to an old militant.

The 400 Russian sailors have been ordered to say nothing. And few people in Saint Nazaire seem inclined to discuss the matter. Laurent Castaing, the CEO of the shipyards, in which the South Korean firm STX now holds a majority share, “will not communicate on the topic”. “This story no longer concerns our town,” says David Samzun, the recently elected socialist mayor. A reception was held on 26 August for the officers of the Smolny, in compliance with protocol, and a list of sporting facilities was provided, but the sailors have not been seen there. Apparently early risers have spotted them boxing and body-building on board.

“They have been asked to keep a low profile and they spend most of their time below decks,” says a Russian nationalist from the Donbas, who now lives just outside Saint Nazaire. Traditionally Russia celebrates Navy Day on 27 July, but there was little sign of any festivities, just a lot of bunting. Some of the sailors danced with officers’ wives visiting for the occasion, but no one went beyond the quayside. Normal life resumed the next day, with walks through the town and swimming in the Loire estuary.

In town, the cadets stand out on account of their extreme youth, blond hair and unbranded T-shirts. They buy cigarettes, have a couple of beers in a bar, pick up a six-pack at the supermarket near the shipyard, but avoid anything stronger. “Vodka here is an outrageous price,” says Mykola, a Ukrainian boilermaker building a cruise liner. At Le Skipper, the nearest brasserie, the sailors go online and Skype their girlfriends back home. Krystof, the Polish proprietor, speaks Russian. He acts friendly but there is “never any mention of the boats”. Even over a drink the Sebass and the Vladi are no-go areas when talk in Saint Nazaire turns to politics. The priority is jobs. “Without the shipyard, Saint Nazaire would just be a dilapidated suburb of [nearby seaside resort] La Baule,” says Jean Rolin, a local writer.

One Sunday in September, a small crowd of about 50 demonstrators gathered on the quay at the stern of the Vladivostok, waving Ukrainian flags and sporting badges marked “#No Mistral for Putin”. They were led by Bernard Grua, a businessman from Nantes, who has been campaigning, almost single-handed, against the sale of the assault ships to Moscow. His supporters know the capabilities of the vessel off by heart. A Mistral can carry 750 soldiers, 16 helicopters, Leclerc tanks, amphibious assault and landing craft, they recite. With Google maps they explore, one by one, Ukraine’s strategic ports. “The Germans flattened your town,” says Grua, for the benefit of the people of Saint Nazaire. “But when the Mistrals attack Mariupol, with Made in France written all over them, the people who didn’t protest will count as collaborators.”

Meanwhile, at the bow of the Vladivostok, at the instigation of the Mistral Gagnons [winning with Mistral] campaign, another group met. The speaker was Jean-Claude Blanchard, a former welder and onetime member of the Trotskyite Workers’ Struggle party and the General Confederation of Labour union. He now heads the far-right Front National faction on Saint Nazaire council. His career is symptomatic of the Front National’s takeover of the docks. He is seconded by Christian Bouchet, the party leader in Nantes. He recently published a French translation of a book by Aleksandr Dugin, an ultranationalist Russian thinker who is purportedly a source of inspiration for Putin.

Among the Mistral Gagnons crowd we spotted a woman wearing the cross of Saint George, symbol of the defenders of Mother Russia. At the other end of the ship there were Lithuanian, Polish and red-and-black Ukrainian nationalist flags. But there were also black-and-white Breton flags fluttering over both crowds. The scene seemed emblematic of the contradictory regional identities that are increasingly evident at a local level but largely ignored at the political centre.

But what can the mayor of Saint Nazaire do, other than take his cue from Hollande’s vacillations and sail as close to the wind as he dares? Everyone here knows that if the Vladivostok is not delivered by 31 October, France will be liable for heavy penalties. Even the local branch of the conservative opposition party dropped plans to demonstrate when someone remembered that it was Alain Juppé, a likely contender in the future presidential race, who inked the Mistral contract. Only one key public figure in Brittany, the owner of the local daily, Ouest France, has voiced his hostility to the deal. “What will happen, if the Vladivostok, which is already fully armed, turns up in the Black Sea, in just a few weeks’ time?” François-Régis Hutin asks. “And what if it one day turns against us?”

This article appeared in Guardian Weekly, which incorporates material from Le Monde