The U.S. Navy says the USS Donald Cook is heading to the Black Sea to conduct maritime security operations and enhance maritime stability with NATO allies in the region.

The Arleigh Burke-class guided-missile destroyer was navigating the Dardanelles Strait in Turkey on Jan. 19 as it headed north toward the Black Sea, the U.S. Navy said in a statement.

“The U.S. Navy routinely operates in the Black Sea consistent with the Montreux Convention and international law,” the statement said.

It did not say when it expected the ship to reach the Black Sea, the Russian Defense Ministry’s Defense Control Center as saying it is “tracking the movements” of the Donald Cook, which according to international convention may stay in the Black Sea for no longer than 21 days.

Tensions in the region have been heightened since Nov. 25 when Russian security forces fired on, boarded, and then seized three Ukrainian vessels near the Kerch Strait, which links the Black Sea and the Sea of Azov.

Moscow claims the Ukrainian vessels illegally entered Russian territorial waters near Crimea, the Ukrainian peninsula that Russia occupied and took over in 2014. It is holding 24 Ukrainian sailors for possible trial on charges of illegal border crossing.

The United States, European Union, and other Western countries have called for their release.

“The United States and the U.S. Navy continue to stand alongside our allies in defense of shared regional interests and maritime stability,” Commander Matthew J. Powel, commanding officer of Donald Cook, said in the Navy statement.

“Our arrival into the Black Sea will showcase the Navy’s interoperability in pursuit of common security objectives, enabling us to respond effectively to future crises or deterring aggression,” he added.

U.S. national security adviser John Bolton said in December that there will be no substantial meeting between President Donald Trump and Russian President Vladimir Putin while Moscow still holds the Ukrainian ships and sailors.

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 at the UN.

Russia also fomented unrest and backed opponents of Kyiv 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.”