As a new report from American intelligence said Russia was not seeking direct conflict with the United States, Biden in a telephone conversation with Putin proposed a summit between the two leaders at a neutral venue in the coming months.

The Russian buildup at the Ukrainian border has caused growing concern in the West in recent days, with the United States saying that troop levels are at their highest since 2014, when war first broke out with Moscow-backed separatists.

Biden "voiced our concerns over the sudden Russian military build-up in occupied Crimea and on Ukraine's borders, and called on Russia to de-escalate tensions," the White House said after the phone call.

White House Press Secretary Jen Psaki said Biden was "not looking for an establishment of trust as much as a predictability and stability" in relations with Russia.

The Kremlin said it would "continue dialogue" without accepting the offer for a first summit with Biden, who came to office vowing a tougher line on Russia including over its alleged interference in US elections and harsh treatment of ailing opposition leader Alexei Navalny.

Putin similarly held a summit in Finland in 2018 with then-US president Donald Trump, who caused a furor at home by appearing to accept the Russian leader's denials of election meddling.

The Biden-Putin call came as Ukrainian Foreign Minister Dmytro Kuleba met in Brussels with top officials of NATO nations including Secretary of State Antony Blinken and the transatlantic alliance's chief, Jens Stoltenberg.

"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," Stoltenberg said at his meeting with Kuleba.

In a clear sign that Washington is looking to bolster its backing for European allies, Defense Secretary Lloyd Austin announced the United States was sending some 500 new personnel to Germany.

- 'We remain vigilant' -

Russian Defense 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, adding the exercises would be completed within two weeks.

Ukraine -- which applied to join NATO 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 and Blinken.

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.

In a new report released Tuesday, the director of US national intelligence said that Russia would "continue destabilization efforts against Ukraine" while concluding that it "does not want a direct conflict" with the United States.

- '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."

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 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."

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