• The Pentagon ordered all U.S. troops in Ukraine to leave the country and reposition elsewhere in Europe.
• The new marching order comes as an estimated 100,000 Russian troops equipped with advanced weaponry line Ukraine's eastern border and northern border with Belarus.
• Meanwhile, the State Department reduced its diplomatic staff at the U.S. Embassy in Kyiv to the "bare minimum."

WASHINGTON – Secretary of Defense Lloyd Austin ordered U.S. troops who deployed to Ukraine last year to leave the country and reposition elsewhere in Europe.

The new marching order comes as an estimated 100,000 Russian troops equipped with advanced weaponry line Ukraine's eastern border and the northern border with Belarus, a Moscow ally.

In November, 160 members of the Florida National Guard, assigned to the 53rd Infantry Brigade Combat Team, deployed to Ukraine to train with local forces.

"The Secretary made this decision out of an abundance of caution — with the safety and security of our personnel foremost in mind — and informed by the State Department's guidance on U.S. personnel in Ukraine," Pentagon spokesman John Kirby wrote in a statement.

"This repositioning does not signify a change in our determination to support Ukraine's Armed Forces, but will provide flexibility in assuring allies and deterring aggression," he added.

Earlier on Saturday, a senior State Department official said that the diplomatic staff at the U.S. Embassy in Kyiv will be reduced to the "bare minimum."

The official, who spoke on the condition of anonymity in order to share details of the State Department's posture in Ukraine, also issued fresh warnings to U.S. citizens that have not yet departed the country.

"It is past time for private citizens to leave Ukraine," explained the senior State Department official.

"American citizens should not expect that the U.S. military is going to rescue them in Ukraine at the last minute. That's not going to be happening in this scenario. And that's why it is past time for them to leave Ukraine," the official added.

"We do a great deal to provide support for our fellow citizens. But as you know, there are real limits to what we are able to do in a war zone," the official said.

The Polish government also appeared to wave entry requirements for U.S. citizens in Ukraine. According to a Buzzfeed news reporter, the State Department sent a notice to Americans in Ukraine late Saturday, once again urging them to leave, saying they could enter Poland without advanced approval.

"Poland has indicated to the U.S. government that U.S. citizens may now enter Poland through the land border with Ukraine. No advanced approval is required," the note said.

Several other nations, including the Netherlands, Australia, Japan and the U.K., are also calling on their nationals to leave the Eastern European country. Dutch airline KLM on Saturday canceled flights to Kyiv indefinitely. The airline cited travel risks and an "extensive safety analysis."

Early morning calls

Several early morning calls were made between Washington and Moscow.

The Pentagon said Austin discussed "Russia's force build-up in Crimea and around Ukraine" with Russian Minister of Defense Sergey Shoygu. Similarly, Secretary of State Antony Blinken, who is currently on diplomatic travel to Fiji, spoke to Russian Foreign Minister Sergey Lavrov and warned that further Russian aggression would be met with a "resolute, massive and united Transatlantic response."

The White House said President Joe Biden's conversation with Russian President Vladimir Putin began at 11:04 a.m. ET and lasted for about an hour.

A White House readout of the call said Biden made it clear that, if Russia undertakes a further invasion of Ukraine, the U.S. and its allies and partners will impose "swift and severe costs on Russia."

Biden said that while the U.S. remains prepared to engage in diplomacy, "we are equally prepared for other scenarios."

On Friday, Biden's national security advisor, Jake Sullivan, urged any Americans still in Ukraine to leave immediately.

Sullivan warned that Putin could order an invasion of its ex-Soviet neighbor "any day now."

For months, the U.S. and its Western allies have watched a steady buildup of Kremlin forces along Ukraine's border with Russia and Belarus. The increased military presence mimics Russian moves ahead of its 2014 illegal annexation of Crimea, a peninsula on the Black Sea, which sparked international uproar and triggered sanctions against Moscow.

The Kremlin has denied that the troop deployment is a prelude to an attack and has instead characterized the movement as a military exercise.

Last month, the Pentagon's top officials warned that the aftermath of a Russian invasion of Ukraine would be "horrific."

"Given the type of forces that are arrayed, the ground maneuver forces, the artillery, the ballistic missiles, the air forces, all of it packaged together. If that was unleashed on Ukraine, it would be significant, very significant, and it would result in a significant amount of casualties," Chairman of the Joint Chiefs of Staff U.S. Army Gen. Mark Milley told reporters at the Pentagon on Jan. 28.

"It would be horrific," added Milley.

Milley, the nation's highest-ranking military officer, said that Russia's posture along Ukraine's border was unlike anything he has seen during his four-decade military career.

He said the Russians have deployed air forces, naval forces, special forces, cyber electronic warfare, command and control, logistics engineers and other capabilities along Ukraine's border.

Amid the Kremlin's deployment, the U.S. and European allies have repeatedly issued threats to impose swift and severe economic consequences if Putin orders an attack on Ukraine.

"He's [Putin] never seen sanctions like the ones I promised," Biden said last month when asked about potential U.S. economic measures. The president said "a disaster" awaits Russia should an attack on Ukraine occur.

Russian officials have repeatedly called on the U.S. to prevent an eastward expansion of the NATO military alliance.

Russia has also demanded that the U.S. "shall not establish military bases" in the territories of any former Soviet states that are not already members of NATO, or "use their infrastructure for any military activities or develop bilateral military cooperation with them."

Since 2002, Ukraine has sought entry into NATO, the world's most powerful military alliance. The group's Article 5 clause states that an attack on one member country is considered an attack on all of them.

The U.S. and NATO have said that such a request cannot be accommodated.

—CNBC's Jessica Bursztynsky contributed to this report.