The United States is relocating its embassy operations in Ukraine from the capital Kyiv to the western city of Lviv, Secretary of State Antony Blinken said on Monday, citing the "dramatic acceleration in the buildup of Russian forces."

The move comes as U.S. officials warn Moscow could launch an attack on Ukraine, including on Kyiv, any day, after amassing more than 100,000 troops close to the eastern border of the former Soviet republic and more forces in Belarus to the north.

Blinken said in a statement the decision was taken out of concern for the safety of embassy staff. Most embassy staff have already been ordered to depart Ukraine and U.S. citizens have been advised to leave the country.

"My team and I constantly review the security situation to determine when prudence dictates a change in posture," Blinken said, adding that Washington would continue working for a diplomatic solution to the tensions.

Embassy operations have been temporarily moved to Lviv – roughly 50 miles (80 kilometers) from Ukraine's western border with Poland – but the embassy would remain engaged with the Ukrainian government in Kyiv, Blinken said.

The United States also said Monday it does not believe Russian leader Vladimir Putin has made a "final decision" on whether to invade Ukraine but he "could move with little or no warning."

Pentagon spokesperson John Kirby also told reporters that U.S. Defense Secretary Lloyd Austin plans to leave for Europe on Tuesday.

Austin will hold meetings at NATO headquarters in Brussels and visit Poland and Lithuania, he said.

Addressing a potential Russian invasion of Ukraine, Kirby said "we still don't believe that some final decision has been made."

"Military action could happen any day," he added. "It is entirely possible that he could move with little to no warning."

State Department spokesperson Ned Price said meanwhile that the United States has seen "no tangible sign of de-escalation" at the Russia-Ukraine border.

"We believe that diplomacy continues to be viable," Price said.

"We believe that there still remains a window to resolve this through dialogue and diplomacy," he told reporters.

At the Pentagon, Kirby separately called out China over its "tacit support" for Moscow in the Ukrainian standoff.

"Their tacit support, if you will, for Russia is deeply alarming and frankly even more destabilizing to the security situation in Europe," he said.

Meanwhile, British Prime Minister Boris Johnson and U.S. President Joe Biden agreed in a call Monday that a "crucial window for diplomacy" remained over the crisis in Ukraine, Johnson's office said.

"They agreed there remained a crucial window for diplomacy and for Russia to step back from its threats towards Ukraine," a Downing Street spokesperson added of the call between the transatlantic allies.

"The leaders emphasized that any further incursion into Ukraine would result in a protracted crisis for Russia, with far reaching damage for both Russia and the world."

The two leaders also stressed that diplomatic discussions with Russia remained "the first priority", and welcomed talks that have already taken place between Russia and NATO allies, according to the spokesperson.

"They agreed that Western allies must remain united in the face of Russian threats, including imposing a significant package of sanctions should Russian aggression escalate," he added.

"They also reiterated the need for European countries to reduce their dependence on Russian gas, a move which, more than any other, would strike at the heart of Russia's strategic interests."

The discussion, the latest among world leaders focused on the crisis in Ukraine, comes after U.S. intelligence officials warned an invasion by Moscow into its western neighbor could be days away. Russia has massed more than 100,000 troops near its Ukrainian border and in neighboring Belarus, where they have been doing joint exercises.

Earlier Monday, Johnson urged Russian President Vladimir Putin to step back from "the edge of a precipice", calling the situation "very, very dangerous" while talking to reporters on a visit to Scotland.

The British leader said he had no plans to visit Moscow, but that he would be discussing the crisis with "various leaders" soon.

Meanwhile, Turkey's flag carrier Turkish Airlines (THY) also announced that "due to the current situation in Ukraine, passengers who have flight tickets purchased before February 13, 2022 (included) and for Turkish Airlines flights arriving to/departing from Ukraine between February 13, 2022 and February 28, 2022 (included), have been granted the additional change and refund rights."