The ships will stay in the region until May 4, the foreign ministry said in a statement. Turkish media said the warships will enter the Black Sea next week.

"We were notified through diplomatic channels 15 days ago... that two US warships will pass toward the Black Sea," a Turkish foreign ministry official told reporters.

The US embassy in Turkey, contacted by AFP, declined comment.

Washington is required to give Ankara a notice of at least 15 days before sending warships through the Bosphorus and Dardanelles straits under the terms of the 1936 Montreux Convention.

The treaty's terms allow foreign warships to stay in the Black Sea for 21 days.

US Navy ships routinely operate in the region in support of Ukraine, which has been fighting Russian-backed forces in its east since a 2014 revolution ousted the pro-Moscow leader in Kiev.

Russia annexed Ukraine's Crimea peninsula around the same time, setting up a confrontation with the West that lingers to this day.

The latest warship deployment comes after Russia sent military reinforcements to both the eastern Ukrainian border and Crimea.

White House press secretary Jen Psaki said on Thursday that the number of Russian troops at the border with Ukraine was now greater "than at any time since 2014".

Ukrainian President Volodymyr Zelensky is due to meet Turkish President Recep Tayyip Erdogan on Saturday in Istanbul.