The Ursa Major, owned by a company that belongs to the Russian defence ministry, sank in international waters off southern Spain in December 2024 with 16 people on board -- two of whom were never found.

The captain finally "confessed" that the ship was transporting "components of two nuclear reactors similar to those used by submarines", reads a government letter to parliament dated February 23 and seen by AFP on Wednesday.

The captain said the reactors "were not carrying nuclear fuel", but this information could not be verified, added the letter, cited in a CNN article published on Tuesday.

According to the US outlet and regional Spanish daily La Verdad de Murcia, the ship may have been sunk in a Western operation because it was taking nuclear reactors to North Korea.

Its owner denounced at the time "a targeted terrorist attack", without providing evidence.

The vessel's official route was from Saint Petersburg to Vladivostok in Russia's Far East.

Questioned by AFP in January, Spanish official Oscar Villar, in charge of investigations at the time, declined to confirm the CNN and La Verdad de Murcia articles, which said they consulted a Spanish investigative report.

Spain's seismology institute had told AFP it had recorded "four seismic signals" near the southeastern coastal city of Cartagena on the day of the sinking.

These were similar to explosions "in quarries to extract construction material, or those carried out by military divers in their anti-mine drills".

The United States in 2022 imposed sanctions on the Ursa Major for providing "transportation services... for the delivery of cargo to Russian-occupied Crimea".