The US embassy in Kyiv warned staff to shelter fearing a 'significant air attack' from Russia after Biden allowed Ukraine to fire long-range missiles deep into Russia.
'The U.S. Embassy in Kyiv has received specific information of a potential significant air attack on November 20,' it said in a message on its website.
'Out of an abundance of caution, the Embassy will be closed, and Embassy employees are being instructed to shelter in place.
The embassy urged US citizens in Kyiv to 'be prepared to immediately shelter in the event an air alert is announced.'
The threat comes just days after Washington lifted the ban on Ukraine using long-range missiles supplied by the US to hit targets in Russia as Putin's forces begin to recover ground.
A short memo advised US citizens in the capital to monitor local media and identify shelters in case of an attack.
Ukraine is bracing for a potential strike after Biden finally gave permission for Kyiv to use its long-range missiles in Russia on Sunday, approaching 1,000 days of war.
The missiles have a range of 190 miles (300km) and could allow Ukraine to repel Russia long-range weapons hitherto capable of striking Ukraine without retaliation.
In anticipation of Ukraine receiving permission to use ATACMS, Russia's air force is said to have redeployed many of its fighter jets and strategic bomber aircraft away from the airbases in range of the missiles to sites further east.
But there are still more than 200 military facilities that could be battered by the US-manufactured munitions from positions along the 800-mile-long frontline.
The decision to concede usage came after months of gridlock in the United States, and followed news North Korean troops were reinforcing Putin's lines in Russia last month.
US-supplied ATACMS missiles will be used against both Russian and North Korean troops deployed against Ukrainian forces in Russia's Kursk region, officials said.
The US said the move was intended to send a message to Russia's ally in North Korea, deploying ground troops for the first time since the end of the Korean War.
Russia has begun to recover ground in recent weeks, hoping to recover momentum as Ukraine's incursion into the Kursk region of Russia slows.
Ukraine's August movement into Kursk was helped along by another US concession, allowing use of some Western weapons to hit targets within Russia in June.
The provision in turn followed Russia's land grab at Kharkiv, reopening its offensive as the West debated allowing use of long-range missiles in May.
The assault was Russia's biggest gain in 17 months, the Telegraph reported, swallowing up 100 sq-miles.
Russian troops made it six miles before Ukraine stabilised the situation.
Western weapons have given Ukraine a lifeline in holding the Russian tide at bay and, more recently, making advances into Russia.
UK-supplied Challenger 2 tanks have been used to buttress Ukraine's surprise incursion into Russia, sources confirmed within days.
The decision to supply Challenger 2s in January 2023 paved the way for Europe and the US to provide Ukraine with Leopard 2 and M1 Abrams tanks.
There had been speculation that Britain could allow use of Storm Shadows to hit targets deep inside Russia in order to open the door for ATACMS.
But the relaxing of Washington's stance on use of ATACMS now opens the possibility of Storm Shadow/SCALP missiles from Britain and France being used.
The long-range missiles use American tech which requires prior approval to be used.
French newspaper Le Figaro initially reported on Sunday that Britain and France had given permission, although confirmation has not yet been announced.
Ukraine had received permission to use the missiles within its own borders.
While ATACMS will give Ukraine significantly improved range to launch attacks on Russian military targets and airbases, they are in short supply on their own.
