Russia's "ongoing militarisation of Crimea, the Black Sea and the Sea of Azov are further threats to Ukraine's independence, and undermine the stability of the broader region," a spokeswoman for NATO chief Jens Stoltenberg said in a statement.

Russian state media reported that Moscow intends to close parts of the Black Sea to foreign military and official ships for six months.

Such a move could affect access to Ukrainian ports in the Sea of Azov, which is connected to the Black Sea through the Kerch Strait, on the eastern tip of the Crimean peninsula annexed by Russia in 2014.

The move has triggered concerns in the United States and the European Union.

The NATO statement echoed that concern, saying: "This would be an unjustified move, and part of a broader pattern of destabilising behaviour by Russia."

It called on Russia to "de-escalate immediately, stop its pattern of provocations, and respect its international commitments".