KIEV (Reuters) - Ukrainian prosecutors said they had searched a vessel in the Black Sea port of Kherson on Saturday as part of an investigation into a suspected illegal delivery of fuel to the Russian navy in annexed Crimea in 2015.

Relations between Kiev and Moscow deteriorated after Russia’s annexation of the Crimea peninsula from Ukraine in 2014 and its backing for pro-Russian separatists in eastern Ukraine. Ukrainian law stipulates that it is illegal to enter Crimean waters without the permission of the Ukrainian authorities.

The Ukraine prosecutors’ office said in a statement on Sunday that the ship, the Maria, had delivered fuel to the Russian Black Sea Fleet in Crimea in 2015 and was now being searched in relation to that delivery.

“The prosecutors office... conducts a search in Kherson on the vessel Maria which is suspected of smuggling oil products,” the prosecutors office said in a statement.

“In June 2015, the same vessel, but under the name Vilga, supplied fuel to the units of the Russian Black Sea Fleet in Sevastopol.”

The prosecutors’ office gave no more details and did not say who owned the vessel. The office could not be reached for further comment.

Last month, Ukraine seized a Russian tanker over its alleged involvement in Russia’s capture of three Ukrainian navy vessels last year.

Russia seized the Ukrainian ships after opening fire on them and is still holding 24 Ukrainian sailors who were on board at the time, accusing them of illegally entering its territorial waters, a charge they deny.

(The story corrects to remove reference to the vessel being Russian owned in lede as ownership unknown.)