The urgent diplomatic drive to defuse the military standoff in Eastern Europe continued on Friday, even as the Pentagon warned that Russia continued to amass more combat troops near its borders with Ukraine, and as President Biden reiterated the United States’ commitment to respond forcefully to a Russian attack.
In a phone call on Thursday with President Volodymyr Zelensky of Ukraine, Mr. Biden “reaffirmed the readiness of the United States along with its allies and partners to respond decisively” if Russia launched a military incursion, according to a White House statement.
The Ukrainian leader, whose government has sought to project calm in the face of American warnings that a Russian attack is imminent, said he thanked Mr. Biden for mobilizing allies to deter aggression from Moscow. But he pointed to prospects of a diplomatic resolution, including an agreement this week with Russia to return to a cease-fire in a long-simmering conflict between Ukrainian troops and Russia-backed separatists in eastern Ukraine.
Russia’s foreign minister, Sergey V. Lavrov, also signaled an opening for diplomacy, saying on Friday that the United States’ response this week to Russian security demands contained “a kernel of rationality” for a possible compromise on issues such as missile deployments and military exercises.
But the Kremlin has said it is pessimistic about prospects for a broader deal, and the United States has described a primary Russian demand, that NATO halt expansion in Eastern Europe, as a non-starter.
The big unknown in the crisis surrounding Ukraine, one of the most worrisome military standoffs in Eastern Europe since the end of the Cold War, are the intentions of President Vladimir V. Putin of Russia, who has maintained a studied public silence on the issue for more than a month.
While Russian leaders have insisted they have no plans to invade Ukraine, Moscow’s buildup of 130,000 soldiers along Ukraine’s borders has prompted the United States and NATO allies to mobilize troops and military assistance for the former Soviet state.
Armored vehicles
Other military or air installations
Russia has begun moving
troops, armor and advanced
antiaircraft systems into
Belarus, a close ally.
Baranovichi
Around 130,000 Russian troops
have been deployed near the
Ukrainian border.
Asipovichy
Marshala Zhukova
Forces deployed north of
Ukraine could stretch
the country’s forces thin and
threaten its capital, Kyiv.
KAZAKHSTAN
Approximate line
separating Ukrainian and
Russian-backed forces.
Persianovskiy
Nearly 20,000 troops are near
two breakaway provinces, where
Ukraine has been locked in a
grinding war with Russian-backed
separatists since 2014.
Rostov-on-Don
SEA OF AZOV
CASPIAN SEA
Armored vehicles
Other military or air installations
Baranovichi
Around 130,000 Russian troops
have been deployed near the
Ukrainian border.
Asipovichy
Marshala Zhukova
Russia has begun moving
troops, armor and advanced
antiaircraft systems into
Belarus, a close ally.
KAZAKHSTAN
Approximate line
separating Ukrainian and
Russian-backed forces.
Persianovskiy
Nearly 20,000 troops are near two
breakaway provinces, where Ukraine
has been locked in a grinding
war with Russian-backed
separatists since 2014.
Rostov-on-Don
SEA OF AZOV
CASPIAN SEA
Armored vehicles
Other installations
Around 130,000 Russian troops
have been deployed near the
Ukrainian border.
Baranovichi
Asipovichy
Russia has begun moving
troops, armor and advanced
antiaircraft systems into
Belarus, a close ally.
KAZAKHSTAN
Approximate line
separating Ukrainian and
Russian-backed forces.
Persianovskiy
CASPIAN SEA
Rostov-on-Don
SEA OF AZOV
Nearly 20,000 troops are near two
breakaway provinces, where Ukraine
has been locked in a grinding
war with Russian-backed
separatists since 2014.
Other installations
Armored vehicles
Around 130,000 Russian troops
have been deployed near the
Ukrainian border.
Russia has begun moving
troops, armor and advanced
antiaircraft systems into
Belarus, a close ally.
Nearly 20,000 troops are near
two breakaway provinces, where
Ukraine has been locked in a
grinding war with Russian-backed
separatists since 2014.
Other installations
Armored vehicles
Note: Numbers for newly arrived troops to Belarus, parts of Crimea, and western Russia are rough estimates.
American and European allies have sought to project a unified front, threatening Moscow with harsh economic sanctions should it attack. But diplomatic efforts have proceeded on multiple tracks, with some leaders quietly pursuing a more conciliatory approach toward Mr. Putin in the hopes of persuading him to draw down his forces.
On Friday morning, President Emmanuel Macron of France spoke by phone with Mr. Putin as part of an effort to de-escalate the crisis.
Mr. Biden and Mr. Putin have not spoken one-on-one since a tense “virtual summit” in early December. The Biden administration called a meeting of the United Nations Security Council on Monday, which could make for a face-to-face debate on Ukraine between the United States and Russia.
The White House still believes there is “a distinct possibility” that Russia could launch an invasion in February, said Emily Horne, spokeswoman for the National Security Council, although American officials do not believe Mr. Putin has decided whether to attack.
On Thursday, the Pentagon, which has ordered 8,500 American troops to be on “high alert” for deployment to Eastern Europe, said that Russia had continued over the past 24 hours to build up “credible combat forces” in western Russia and in Belarus, near the borders with Ukraine.
“We still believe there’s time and space for diplomacy,” said the Pentagon spokesman, John Kirby. “But thus far, it has not achieved the kind of results that the international community would like to see.”