Moscow retorted that it had sent troops to its western borders for combat drills because of "threats" from the transatlantic alliance, in an increasingly heated war of words between Russia and the West.

As concern mounted, Kiev's Foreign Minister Dmytro Kuleba held talks with Stoltenberg and US Secretary of State Antony Blinken in Brussels.

"In recent weeks, Russia has moved thousands of combat-ready troops to Ukraine's borders, the largest massing of Russian troops since the illegal annexation of Crimea in 2014," Stoltenberg said at his meeting with Kuleba.

"Russia's considerable military build-up is unjustified, unexplained and deeply concerning -- Russia must end this military build-up in and around Ukraine, stop its provocations and de-escalate immediately."

Russian Defence Minister Sergei Shoigu hit back that Moscow has deployed troops to its western borders for "combat training exercises" in answer to NATO military moves.

"In response to the alliance's military activities that threaten Russia, we took appropriate measures," Shoigu said in televised remarks.

He said the troops "have shown full readiness" and that the exercises would be completed within two weeks.

- 'We remain vigilant' -

Ukraine -- which applied to join the US-backed alliance in 2008 -- is pressing Western powers for "practical" support as it seeks to deter any new aggression from Moscow.

"Russia will not be able to catch anyone by surprise anymore. Ukraine and our friends remain vigilant," Kuleba said as he met Stoltenberg.

"We do not and will not lose time, and should Moscow take any reckless move or start a new spiral of violence it will be costly in all senses."

In Brussels, Kuleba also met Blinken, who has jetted back for fresh talks at NATO after a visit last month.

Blinken insisted "the US stands firmly behind the sovereignty and the territorial integrity of Ukraine" and said the country's "Euro-Atlantic aspirations" would be discussed in the alliance.

Some NATO members remain deeply reluctant to heed Ukraine's appeal to speed up its push for admission as they seek to avoid ratcheting up the tension.

- 'Powder keg' -

The Kremlin has warned NATO off getting further involved in Ukraine and on Tuesday accused the alliance members of turning the country into a "powder keg".

"If there is any aggravation, we of course will do everything to ensure our security and the safety of our citizens, wherever they are," Deputy Foreign Minister Sergei Ryabkov was quoted as saying by Russian news agencies.

"But Kiev and its allies in the West will be entirely responsible for the consequences of a hypothetical exacerbation," he added.

The troop build-up comes amid a spike in violence along the front line between Russian-backed separatists and Ukrainian forces in the east of the country.

Ukraine's military said that one of its soldiers was killed on Tuesday when a drone dropped grenades on his position, taking to 29 the number of fatalities for Kiev this year.

Fighting subsided in 2020 as a ceasefire agreement took hold last July, but clashes have picked up again since the start of the year, with each side blaming the other.

Analysts say that NATO allies are not willing to commit their own forces to a conflict against Moscow over Ukraine, but could bolster other forms of assistance.

"NATO members will not send any boots on-the-ground in Ukraine," said Bruno Lete, a senior fellow at the German Marshall Fund of the United States think tank.

"Their support will be limited to political messaging, military advice, and technical aid."

But in a clear sign that Washington is looking to bolster its military backing for European allies, US Defense Secretary Lloyd Austin announced it was sending some 500 new personnel to Germany.

Austin said during a visit to Berlin that the US government wanted to put its security cooperation with Germany and NATO on a stronger footing after four years of tense relations under Donald Trump.