Hi-tech and highly sensitive equipment has reportedly disappeared from a warship France has built and is refusing to hand over to the Russians.

The vessel, the Vladivostok, has been undergoing trials off the Atlantic port of Saint-Nazaire, where it was built, with a 400-strong Russian crew. The amphibious assault ship, one of two Mistral helicopter carriers being built by France for the Russian military in a €1.2bn (£955m) deal with Moscow, was scheduled to be delivered by the end of this month.

According to French reports, the “sensitive material” was reported missing on Tuesday, the same day the French president, François Hollande, said the ship’s delivery to Moscow would be postponed “until further notice” because of concerns over Russia’s involvement in the Ukraine conflict.

Le Point magazine reported that police at Rennes had opened a “preliminary investigation” into the alleged theft of hard drives from the ship’s computers as well as sensitive and state-of-the-art communications equipment installed by electronics firm Thales. Officers said there was no evidence of a break-in.

The French defence ministry said the investigation was concentrating on those in the “immediate vicinity” of the vessel. “We haven’t got to the point of suspecting the Russians,” a defence spokesperson said.

According to the Russian news agency Interfax, Moscow had hoped the Vladivostok would sail from Saint-Nazaire on Thursday or Friday. The 400 Russian sailors who have been in the French port since the end of June for training had already transferred their belongings from the Russian ship where they were sleeping to the Vladivostok, which was moved last weekend from the shipyard in Saint-Nazaire to a nearby Loire estuary.

A second Mistral, named the Sevastopol, after the Black Sea port in the Crimea that Russia has annexed from Ukraine, is under construction in Saint-Nazaire and is scheduled for delivery next year.