The United States on Friday told its citizens in Ukraine to leave the country within 48 hours amid growing concerns that Russia could attack the former Soviet member country, reported AFP.

An attack by Russia “could occur any day now”, White House National Security Advisor Jake Sullivan told reporters in Washington.

“If a Russian attack on Ukraine proceeds, it is likely to begin with aerial bombing and missile attacks that could obviously kill civilians,” he said. “Any American in Ukraine should leave as soon as possible, and in any event in the next 24 to 48 hours.”

On Saturday, a spokesperson for the Russian foreign ministry said the country has decided to “optimise” its diplomatic staff in Ukraine, reported AFP. Maria Zakharova added that this was being done fearing “provocations” by Kyiv or its allies.

Since early this year, Russia has amassed close to 1.3 lakh troops at the Ukrainian border, according to the New York Times. The two countries have been engaged in a conflict since 2014, when Russia annexed Ukraine’s Crimean Peninsula and backed separatist rebellions in the country’s eastern regions of Donetsk and Luhansk.

However, Russia has denied the speculation that it would invade Ukraine.

On Friday, Sullivan said that the US was seeing signs of escalation from Russia, including new forces arriving on the Ukrainian border, reported Reuters.

He also dismissed speculation that Moscow would not invade while the Beijing Olympics were being held. Sullivan said such an attack “could occur” before the Games end on February 20.

The scenario of an imminent attack is “a very, very distinct possibility,” he added.

Sullivan made the statement shortly after President Joe Biden and six European leaders, the heads of North Atlantic Treaty Organization, or NATO, and the European Union held talks on the crisis. The European countries have also stepped up the warning of the possible invasion.

The Pentagon, the US’ Department of Defense, announced it was sending 3,000 more troops to ally Poland. Earlier this month, the US had deployed 3,000 troops in Europe. Pentagon spokesperson John Kirby had said that troops would not be fighting in Ukraine, but were being sent to support members of the NATO.

After holding the talks, German Chancellor Olaf Scholz’s spokesperson said the aim was to prevent a war in Europe. But if Moscow fails to pull back, she said “the allies are determined to jointly take swift and deep sanctions against Russia.”

European Union chief Ursula von der Leyen said these sanctions would target Russia’s financial and energy sectors.

Australia and New Zealand have now become the latest countries to urge their citizens to leave Ukraine as soon as possible, after the United Kingdom, Japan, Latvia, Norway and the Netherlands.

Meanwhile, Russia Deputy United Nations Ambassador Dmitry Polyanskiy mocked Sullivan’s comments.

“Some reasonable people were hoping US-fanned hysteria was waning,” he tweeted. “Maybe they put a jinx on it, because scaremongers have clearly got second wind. Our troops are still on our territory and I wonder if the US will invade Ukraine itself – someone has to, after such a panic campaign.”

On Thursday, Russia had sent its tanks and troops across Belarus – which shares it borders with Russia and Ukraine – for live fire drills. While Russia and Belarus have not disclosed the number of troops participating in the drills, the US has said about 30,000 soldiers were being dispatched.

Russia also sent six warships through the Strait of Istanbul, for naval drills. The strait is an important waterway, located in northwest Turkey, that forms part of the continental boundary between Asia and Europe.

Russia wants guarantees from the West, including that no missile will be deployed near its borders, that Ukraine would not get a NATO membership and the alliance’s military infrastructure will be scaled back.

The West has not agreed to the demands but has said it was willing to talk about arms control. Russia, however, said the response was disrespectful.