A Moscow court on February 12 upheld the extension of the pretrial detention of four of 24 Ukrainian sailors detained by Russian forces along with their three naval vessels in November near the Kerch Strait, which links the Black Sea and the Sea of Azov.
Last month, the Lefortovo court in Moscow extended the pretrial detention of the sailors, who are charged with illegal border crossing, until April 24 and April 26.
On February 7, the court rejected the appeals lodged by four of the sailors against the extension of their pretrial detentions.
The appeals of the other 16 Ukrainian sailors from the group will be assessed in the coming days.
Russia has held the Ukrainian sailors since its forces fired on, boarded, and seized their vessels near the Kerch Strait on November 25.
Moscow claims the Ukrainian vessels illegally entered Russian territorial waters near Crimea, the Ukrainian peninsula that Russia occupied and took over in 2014.
The sailors face up to six years in prison if convicted.
The United States and other Western countries have called for the Ukrainian sailors' release, calling their detainment illegal.
Russia moved swiftly to seize control over Crimea after Moscow-friendly Ukrainian President Viktor Yanukovych was pushed from power in Kyiv by the pro-European Maidan protest movement in February 2014.
Putin's government sent troops without insignia to the peninsula, seized key buildings, took control of the regional legislature, and staged a referendum denounced as illegitimate by at least 100 countries.
Russia also fomented unrest and backed separatists in eastern Ukraine, where more than 10,300 people have been killed in the ensuing conflict since April 2014.
The International Criminal Court (ICC) ruled in November 2016 that the fighting in eastern Ukraine is "an international armed conflict between Ukraine and the Russian Federation."