Russia's buildup of military personnel threatening Ukraine probably totals up to 190,000, the United States ambassador to the Organization for Security and Co-operation in Europe (OSCE) said during a meeting on the Ukraine crisis on Friday.

"We assess that Russia probably has massed between 169,000-190,000 personnel in and near Ukraine as compared with about 100,000 on Jan. 30," Michael Carpenter told the meeting, which Russia did not attend. "This is the most significant military mobilization in Europe since World War II."

U.S. Defense Secretary Lloyd Austin, while in Poland on Friday, warned that a Russian invasion of Ukraine could trigger an exodus of people fleeing toward the European Union.

"If Russia further invades Ukraine, Poland could see tens of thousands of displaced Ukrainians and others flowing across its border trying to save themselves and their families from the scourge of war," he said alongside his Polish counterpart Mariusz Blaszczak in Warsaw.

Blaszczak said Poland was ready to help those forced to leave Ukraine in the event of a Russian attack. Last week, Poland's interior minister said his country was preparing emergency shelters.

The U.S. recently moved 4,700 troops from the 82nd Airborne Division from North Carolina to Poland to bolster the 4,500 U.S. forces already stationed in the country. Austin was also due to meet President Andrzej Duda while in Warsaw.

Meanwhile, Russian President Vladimir Putin welcomed his close ally Belarusian strongman Alexander Lukashenko to Kremlin on Friday, announcing the leaders would discuss military cooperation and oversee the upcoming war games.

Lukashenko's visit to Russia comes as Moscow is up against the West over European and Ukrainian security, tensions escalated after large-scale Russian drills in Belarus.

"We will, of course, talk about the situation in the region, assess how military cooperation is going, including the ongoing military drills," Putin told Lukashenko ahead of talks.

"Tomorrow, we will even participate in one of the most significant events in this complex of military cooperation," the Russian leader said.

The Russian Defense Ministry announced earlier Friday that Putin would oversee military exercises the following day that would involve the launch of cruise and ballistic missiles.

Belarus, wedged between Russia and EU member states, is more closely aligned with Russia in the wake of historic anti-Lukashenko protests in 2020.

"Our Western partners – as you call them – have brought the military-political spectrum to the forefront, and we have to react to it, including by holding military exercises and through diplomacy," Lukashenko told Putin in Moscow.

He also accused Western leaders of "scaring the world by saying that 'tomorrow' we will attack, encircle, destroy Ukraine."

Washington has estimated that some 30,000 Russian troops have been deployed at neighboring Belarus as part of joint exercises that are due to run until Sunday.

Those drills as well as other large-scale exercises near Ukraine have fueled concerns in European capitals and Washington that Moscow is preparing an attack on its neighbor.

Russia has denied any plans and accused Ukraine of breaching cease-fire agreements in the east of the country, where the army is fighting pro-Moscow separatists.

Lukashenko said Thursday his country could host nuclear weapons if it faces any external threats, as tensions soar between his ally Putin and Western leaders.