Russia and Ukraine held simultaneous military drills on Wednesday as NATO foreign and defence ministers began emergency discussions on a massing of Russian troops near the Ukrainian border.
Washington and NATO have been alarmed by the large build-up of Russian troops near Ukraine and in Crimea, the peninsula that Moscow annexed from Ukraine in 2014, and two US warships are due to arrive in the Black Sea this week.
Russia -- which said the US naval move was an unfriendly provocation and warned Washington to stay far away from Crimea and its Black Sea coast -- says the build-up is a three-week snap military drill to test combat readiness in response to what it calls threatening behaviour from NATO. It has said the exercise is due to wrap up within two weeks.
Ahead of the arrival of the US warships, the Russian navy on Wednesday began a drill in the Black Sea that rehearsed firing at surface and air targets. The drill came a day after NATO Secretary-General Jens Stoltenberg called on Moscow to end its troop build-up.
In Ukraine, armed forces rehearsed repelling a tank and infantry attack near the border of Russian-annexed Crimea while its defence minister, Andrii Taran, told European parliamentarians in Brussels that Russia was preparing to potentially store nuclear weapons in Crimea.
Taran provided no evidence for his assertion but said Russia was massing 110,000 troops on Ukraine's border in 56 battalion-sized tactical groups, citing Kyiv's latest intelligence.
Fighting has increased in recent weeks in eastern Ukraine, where government forces have battled Russian-backed separatists in a seven-year conflict that Kyiv says has killed 14,000 people.
US Secretary of State Antony Blinken, who held talks in Brussels with Stoltenberg ahead of a video conference of all 30 NATO allies, said the alliance would "address Russia's aggressive actions in and around Ukraine", without elaborating.
Russia's relations with the United States slumped to a new post-Cold War low last month after US President Joe Biden said he thought Vladimir Putin was a "killer".
In a phone call with Putin on Tuesday, Biden proposed holding a summit between the estranged leaders to tackle a raft of issues, including reducing tensions over Ukraine.
The Kremlin on Wednesday said it was too early to talk about such a summit in tangible terms and that holding such a meeting was contingent on Washington's future behaviour, in what looked like a thinly veiled reference to potential US sanctions.
Russia has regularly accused NATO of destabilising Europe by bolstering its troops in the Baltic countries and Poland - all members of the Atlantic alliance - in the wake of Moscow's annexation of Crimea.
NATO has denied a claim by Russian Defence Minister Sergei Shoigu that the alliance was deploying 40,000 troops and 15,000 pieces of military equipment near Russia's borders, mainly in the Black Sea and the Baltic regions.