The United States Embassy in Kyiv will reopen tomorrow after closing due to the threat of an air attack, the State Department has said.
Spokesperson Matthew Miller declined to say what kind of threat had forced the embassy to shut down as a safety precaution.
"We take the safety and security of our personnel ... extremely seriously," Mr Miller said.
The US mission had closed a day after Russia vowed to respond to Ukraine's firing of long-range US-supplied missiles at Russian territory for the first time in the nearly three-year war.
The United States said it shut its embassy in Kyiv due to what it called the threat of a significant air attack.
Following this announcement and after an air raid siren in the early afternoon jangled nerves in the capital, Ukraine's military spy agency said Russia was trying to sow panic by circulating fake online messages about a massive looming missile and drone attack.
"The enemy, unable to subdue Ukrainians by force, resorts to measures of intimidation and psychological pressure on society. We ask you to be vigilant and steadfast," it said.
A US government source said the embassy closure was "related to ongoing threats of air attacks".
The Italian and Greek embassies said they too had closed their doors. The French embassy remained open but urged its citizens to be cautious.
Ireland's Department of Foreign Affairs has said staff at the Irish Embassy in Kyiv are working from home "as a precautionary measure".
Yesterday, Ukraine used US ATACMS missiles to attack an arms depot inside Russia, making use of newly granted permission from the outgoing administration of US President Joe Biden on the 1,000th day of Russia's full-scale invasion of Ukraine.
Russia has been signalling to the US and its allies for weeks that if they give permission to Ukraine to strike deep into its territory with Western-supplied missiles then Russia will consider it a major escalation.
"Out of an abundance of caution, the embassy will be closed, and embassy employees are being instructed to shelter in place," the US Department of State Consular Affairs said in a statement on the embassy's website.
"The US Embassy recommends US citizens be prepared to immediately shelter in the event an air alert is announced."
Russia said it had no comment.
Russian foreign intelligence chief Sergei Naryshkin said in an interview published that Russia would retaliate against NATO countries that facilitate long-range Ukrainian missile strikes against Russian territory.
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Psychological operation
Earlier, the Ukrainian air force told people to seek shelter because of a missile threat. Senior officials told people not to ignore the alert.
That came shortly before Ukraine's top military spy agency GUR issued a warning about a Russian psychological operation it said involved fake messages claiming to have been sent by the agency.
"A message is being spread via messengers and social networks ... about the threat of a 'particularly massive' missile and bomb strike on Ukrainian cities today," GUR said in a statement.
Two Ukrainian military personnel told Reuters they received messages informing them that Russia would launch more than 300 drones and also use warships, warplanes and ground-based systems to fire missiles in a huge salvo.
Reuters could not immediately determine how the messages were sent. One soldier said he received one from a friend.
The war is at a volatile juncture, with nearly a fifth of Ukrainian territory in Russian hands, North Korean troops deployed in Russia's Kursk region, and doubts over the future of aid from Western countries as US president-elect Donald Trump returns to the White House.
On Sunday, Russia staged a massive missile and drone strike on Ukraine's national power grid that killed seven people and renewed fears over the durability of the hobbled energy network.
The GUR spy agency said earlier that a Russian military command post had been "successfully struck" in the town of Gubkin in Russia's Belgorod region, around 168km from the border with Ukraine.
The statement did not specify who carried out the attack, when it took place or the type of weapon used.
Ukraine has also used drones for deep strikes against targets in Russia.
Bloomberg later cited a Western official as saying Ukraine had fired British Storm Shadow missiles into Russia.
A spokesperson for UK Prime Minister Keir Starmer said his office would not be commenting on reports or operational matters. There was no immediate comment from Ukraine.
'Persistent Russian attacks'
The embassy in Kyiv urged US citizens in Ukraine to have reserves of water, food and other essentials such as required medications for the event of a "possible temporary loss of electricity and water" caused by Russian strikes.
"Persistent Russian attacks targeting civilian infrastructure throughout Ukraine may result in power outages, loss of heating, and disruption of municipal services," it said.
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Yesterday, Russian President Vladimir Putin lowered the threshold for a nuclear strike in response to a broader range of conventional attacks.
The US said afterwards it had not seen any reason to adjust its nuclear posture.
Commenting on the US embassy warning about an attack, Andriy Kovalenko, head of the Ukrainian Security Council's Centre for Countering Disinformation, said Russia was ready to conduct more conventional missile strikes.
"Let me remind you that the Russians have been stockpiling missiles for a series of attacks on Ukraine for months. This includes Kh-101 missiles, which they continue to produce, as well as Kalibrs and ballistics," he said.
