The US embassy in Kyiv warned staff to shelter over a 'significant air attack' from Russia after President Joe 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.
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.