With the large Russian troop buildup on the borders of Ukraine, White House, NATO and European leaders have been working to project a united front to deter the Kremlin and convince President Vladimir V. Putin of Russia that there would be a heavy price to pay in the event of war.
On the eve of his virtual summit with Mr. Putin, President Biden spoke with allied leaders including President Emmanuel Macron of France, Chancellor Angela Merkel of Germany, Prime Minister Mario Draghi of Italy and Prime Minister Boris Johnson of Britain.
“The leaders discussed their shared concern about the Russian military buildup on Ukraine’s borders and Russia’s increasingly harsh rhetoric,” according to a readout from the White House. They called on Russia to de-escalate tensions, the statement said, and agreed that diplomacy was the only way forward.
Even as the leaders issued assurances of solidarity, many European officials are worried that Mr. Putin could respond to pressure by diminishing gas supplies to Europe as winter approaches.
The crisis also comes as Germany is changing leadership and France is facing an election, creating a shifting landscape that the White House fears the Kremlin might move to exploit.
At the Pentagon on Monday, Defense Secretary Lloyd J. Austin III convened top military and civilian officials, including Gen. Mark A. Milley, the chairman of the Joint Chiefs of Staff, and Gen. Tod D. Wolters, the head of the military’s European Command, to discuss the Russian troop buildup.
Officials said that there was an effort underway to send additional defensive weapons, including anti-tank Javelins, to Ukraine, but that they may be kept in position outside the country to avoid giving Mr. Putin a pretext for military action.
Some NATO country officials have counseled against such a move, suggesting that it would not make much difference in the outcome if an invasion should come and could provoke Mr. Putin, rather than deter him.
Given the risks involved, some analysts, like Dmitri V. Trenin, director of the Carnegie Moscow Center, argue that Mr. Putin is seeking diplomatic leverage and that the main aim of his troop buildup is not to conquer Ukraine but to get a satisfactory response from the United States.
“Putin is clearly frustrated about being ignored by Washington,” Mr. Trenin said. He noted that Mr. Putin’s earlier troop buildup near Ukraine secured him an early summit meeting with Mr. Biden in June. “This time it will bring Biden to a dialogue on Ukraine and other security issues in Europe.”
But Mr. Trenin said the use of force could not be ruled out.
John F. Kirby, the chief Pentagon spokesman, declined to say whether the United States still had a small number of military advisers in Ukraine or whether the administration had decided to send additional military assistance there.
The Ukrainian defense minister, Oleksii Reznikov, warned that “a major war in Ukraine would plunge the whole of Europe into crisis.”
In an open letter to the Atlantic Council, he said that among other major concerns for European society, a war could result in “between three and five million Ukrainian refugees fleeing the Russian invasion” and could seriously disrupt food imports, including of grain, “creating a whole range of food security problems for the entire continent.”
“Russia’s attack would also mark a definitive end to the rules-based international order, opening the door to a new era of global insecurity that would threaten decades of unprecedented peace throughout most of Europe,” he wrote.