The Russian vessel Urengoi has been unlawfully working in the Kerch strait for days, Volodymyr Ogryzko, a Ukrainian senior deputy foreign minister, told reporters in Kiev.

"The ship has entered Ukraine's territorial waters, violated our sovereignty and is conducting works on deepening the seabed," he said.

Ukraine "has demanded an immediate stop to the works," he said.

It was not clear whether the ship was state-owned or a private vessel and there was no immediate response from Russian officials to the Ukrainian charge.

Russia and Ukraine have yet to demarcate a border in the Kerch strait, a narrow waterway between the Sea of Azov to the Black Sea that is a shipping gateway to the Mediterranean and separates Ukraine's Crimea from Russia's Krasnodar region.

An argument over the strait's Tuzla island brought the two nations to the brink of a full-blown confrontation in September 2003, after Russia began construction of a dike from its coast to the island.

It is said that whoever controls the island controls the lucrative shipping traffic in the strait.

Tuesday's statement comes against the background of a simmering dispute over whether a lease agreement on Russia's Black Sea fleet, which is based in Crimea, allows Moscow to have jurisdiction over navigation installations on the peninsula.

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