Earlier on Tuesday, Moscow's Lefortovo district court extended the detention term for 12 captive Ukrainian sailors until April 24.

Russia must release Ukraine’s sailors immediately, not use them as political pawns, the U.S. Embassy in Ukraine wrote on Twitter Tuesday.

"For over 50 days, Russia has unjustly detained Ukrainian sailors who were exercising Ukraine’s freedom of navigation in its territorial waters," the American diplomats wrote. "We are deeply concerned about their health & rights."

Russia must release Ukraine’s sailors immediately, not use them as political pawns. For over 50 days, Russia has unjustly detained Ukrainian sailors who were exercising Ukraine’s freedom of navigation in its territorial waters. We are deeply concerned about their health & rights. — U.S. Embassy Kyiv (@USEmbassyKyiv) January 15, 2019

As UNIAN reported earlier on Tuesday, Moscow's Lefortovo district court extended the detention term for 12 captive Ukrainian sailors until April 24.

UNIAN memo. On the morning of November 25, Russia blocked the passage to the Kerch Strait for the Ukrainian tugboat "Yany Kapu" and two armored naval boats "Berdyansk" and "Nikopol," which were on a scheduled re-deployment from the Black Sea port of Odesa to the Azov Sea port of Mariupol.

The Ukraine Navy Command noted that the Russian side had been informed of the plans to re-deploy the vessels in advance in accordance with international standards to ensure the safety of navigation.

The Russian coast guard ship "Don" rammed the Ukrainian tugboat, damaging the Ukrainian vessel. As the Ukrainian boats were heading back in the Odesa direction after being rejected passage via the Kerch Strait, Russian coast guards opened aimed fire on them.

Read alsoEU urges Russia to immediately release Ukrainian sailors, Pavlo Hryb, Edem Bekirov, other political prisoners

All 24 crew members on board were captured and later remanded in custody for two months, being charged with "illegal border crossing" (the sailors are facing up to six years in prison).

Three crewmen were wounded in the attack. Russian-controlled "courts" in occupied Crimea ruled that all 24 detainees should be remanded in custody, after which they were transferred to the Moscow-based Lefortovo and Matrosskaya Tishina detention centers.

Ukrainian Foreign Minister Pavlo Klimkin says the Russian Federation will exploit the issue of the release of Ukrainian sailors and political prisoners captured by Moscow in the context of the upcoming Ukraine elections.

On Jan 8, 2019, Ukrainian Foreign Minister Pavlo Klimkin said Russia would exploit the issue of the release of Ukrainian sailors and political prisoners captured by Moscow in the context of the upcoming Ukraine elections.