A number of nations advised their citizens to leave Ukraine as the United States raised alarm about a possible move by Russia to invade, a claim that Moscow has rejected.

Sweden and Spain urged nationals to leave as soon as possible in light of the "changing security situation" Saturday while Italy's Foreign Ministry also asked its citizens to leave the country temporarily.

The countries are following others such as Germany, the Netherlands and Britain, which previously made a similar appeal to their compatriots in the Eastern European country.

The embassy of Saudi Arabia in Kyiv on Saturday also called on its citizens to quickly get in touch to facilitate their departure from the country.

Turkey joined other countries in issuing a travel warning for Ukraine Saturday in view of growing tensions between the West and Russia.

The United States said on Saturday it was ordering most staff at its embassy in Kyiv to leave Ukraine immediately due to the threat of an invasion by Russia.

An updated travel advisory said the State Department had ordered the departure of most employees at the embassy in Kyiv, adding to its call earlier this week for private U.S. citizens to leave Ukraine immediately.

A senior State Department official said the Biden administration would continue diplomatic efforts to "ensure that Ukraine does not become a war zone."

Washington's warnings that the Russian buildup of more than 100,000 troops on Ukraine's borders could herald an invasion have become increasingly urgent in recent days. Moscow has denied its plans to invade, saying it has its own security concerns over what it terms aggression by NATO allies.

"It appears increasingly likely that this is where this situation is headed – towards some kind of active conflict, and that is why we are reducing our staff to a bare minimum while we still have the ability to get our official people out safely and in a predictable fashion," said the official, who briefed reporters on condition of anonymity as reported by Reuters.

A string of other countries including Britain, Japan and Australia have also said their citizens should leave.

Russia, which has accused Western nations of spreading lies, meanwhile said on Saturday it had decided to "optimize" its diplomatic staff numbers in Ukraine, fearing "provocations" by Kyiv or another party.

Moscow did not say whether that meant a reduction in staff numbers but said the embassy and consulates in Ukraine continued to perform their key functions.

On Friday, the U.S. government said that it considers a Russian incursion into Ukraine possible before the close of the Winter Olympics in China on Feb. 20.

Moscow denies having any such plans, but has massed over 100,000 troops along Ukraine's borders.

Instead, the Kremlin has used the attention on the region to express fears that NATO has encroached too close to its territory and demanded that the security alliance withdraw from what Russia considers its sphere of influence.

In response to the tensions, NATO countries have deployed troops and equipment to fellow member states that share a border with Ukraine.

The Romanian Defense Ministry announced late Friday that the U.S. Air Force has transferred eight F-16 fighter jets to its territory. The aircraft arrived at Borcea Air Base, 150 kilometers (93 miles) east of Bucharest.

They would take part in joint exercises with the Romanian military along with 150 U.S. soldiers, the ministry said in a statement.

U.S. President Joe Biden and Russian President Vladimir Putin are set to discuss the stand-off in a phone call on Saturday.

A number of countries have advised their citizens to leave Ukraine – including Russia.

Russian Foreign Ministry spokesperson Maria Zakharova justified the new guidance by pointing to "possible provocations by the Kyiv regime or third states."

At the same time, Zakharova reproached the West and Ukraine. "Considering the significant influence that Washington and London have on Kiev ..., we draw the conclusion that our American and British colleagues seem to be aware of preparations for military actions in Ukraine that could significantly complicate the situation in the security sphere."

Both the Russian embassy and consulates in Ukraine continued to carry out their basic activities, Zakharova stressed.

The United States and Britain were among the first countries to tell their citizens to leave Ukraine as quickly as possible.

New Zealand does not have diplomatic representation in Ukraine, Foreign Affairs Minister Nanaia Mahuta said in a statement. Wellington's ability to provide consular assistance to citizens in Ukraine was therefore "very limited," she added.

Israel also issued a warning against travel to Ukraine. The Israeli Foreign Ministry said late Friday that the relatives of diplomats are to be evacuated from the country and instructed all Israeli citizens living there to register via a link.

The European Union's delegation in Ukraine was "not evacuating," EU foreign service spokesperson Peter Stano said on Saturday. However, "non-essential staff (have) been given the opportunity to telework from outside the country," he told Deutsche Presse-Agentur.