The case focuses on the fate of the captured sailors, who were manning three Ukrainian vessels off Russian annexed Crimea when Russian ships fired on and seized their boats.

The International Tribunal for the Law of the Sea, in the German city of Hamburg, said it would announce its decision on May 25.

Moscow has said it does not recognise the court's jurisdiction.

Addressing the tribunal, Ukraine's deputy foreign minister Olena Zerkal called for the immediate release of the sailors.

"From the moment of the detentions, Ukraine has worked urgently to solve this matter," she said.

She described their detention as "an additional illustration of Russia's continued disrespect for international law".

"Each additional day of detentions, each interrogation, each court case aggravates the dispute between the parties," she said.

Russia has accused the sailors of violating its maritime borders.

Moscow did not send representatives to the hearing.

Outgoing Ukrainian President Petro Poroshenko has insisted the sailors are "prisoners of war" and described their detention as "blatant proof that Russia continues to show cynical disrespect for human rights".

"We expect that in a matter of weeks the Tribunal will oblige Russia to release our sailors and Ukrainian ships, and its actions will be considered illegitimate," Poroshenko said in a statement last month.

"Russia still has a chance to free our sailors and ships without waiting for the formal recognition of the international crime committed by the Kremlin."

The Ukrainian sailors face up to six years in prison for illegally crossing Russian borders, lawyers have said.

Critics of the Kremlin have accused Russia of preparing a show trial.

The capture of the Ukrainian ships was the most dangerous direct clash in years between the ex-Soviet neighbours.

They have been locked in a confrontation since 2014 when Russia annexed Crimea and supported an insurgency in eastern Ukraine.

Some 13,000 people have been killed in the conflict in eastern Ukraine, according to the UN.