Ukraine and the United States launched joint naval exercises in the Black Sea on Monday, in a show of US cooperation with Kiev as it faces off with Russia.

Twenty-six ships from eight nations were taking part in the annual Sea Breeze exercises co-hosted by the two nations, including the USS Porter guided-missile destroyer.

"The Black Sea is a vital waterway that is critical to maritime commerce and stability in Europe," the commander of the US Sixth Fleet, Vice Admiral Eugene Black, said in a telephone press briefing on the drills. 

"The US, NATO allies, and partner nations recognise the importance... of being able to seamlessly operate together and deter adversary aggression," he said.

Washington is a key ally of Ukraine in its conflict with Russia over Crimea and pro-Moscow separatist regions in eastern Ukraine. 

Russia annexed Crimea from Ukraine in 2014, and in 2018 Russian forces boarded and took control of three Ukrainian naval ships off the Black Sea peninsula.

The exercises, which have taken place 20 times since 1997, were to last a week and be limited to sea and air operations because of coronavirus restrictions.

Bulgaria, Georgia, Norway, Romania, Spain and Turkey were also taking part.

President Vladimir Putin was in Crimea Monday to visit a shipyard and news agency Interfax quoted Russia's supreme command as saying the country's Black Sea fleet was monitoring US ships in the region.