The United States shut its embassy in Kyiv on Wednesday due to "specific information of a potential significant air attack" and told its citizens in Ukraine to be ready to swiftly seek shelter.
The Italian and Greek embassies said they had also closed their doors after the unusual U.S. warning, while the French embassy remained open but urged its citizens to be cautious.
Ukraine used U.S. ATACMS missiles Tuesday to attack an arms depot inside Russia, making use of newly granted permission from the outgoing administration of President Joe Biden on the 1,000th day of Russia's full-scale invasion of Ukraine.
Russia has been signaling to the United States and its allies for weeks that if they give permission to Ukraine to strike deep into Russian territory with Western-supplied missiles, then Moscow 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 U.S. Department of State Consular Affairs said in a statement on the embassy's website.
"The U.S. Embassy recommends U.S. citizens be prepared to immediately shelter in the event an air alert is announced."
The Kremlin said it had no comment. Russian foreign intelligence chief Sergei Naryshkin said in an interview published on Wednesday that Moscow would retaliate against NATO countries that facilitate long-range Ukrainian missile strikes against Russian territory.
Ukraine's military intelligence agency said Wednesday that a Russian military command post had been "successfully struck" in the town of Gubkin in Russia's Belgorod region, around 168 km (105 miles) from the border with Ukraine.
The Ukrainian statement raised the possibility of a second ATACMS strike, but 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.
The war is at a volatile juncture, with a fifth of Ukrainian territory in Russian hands, North Korean troops deployed in Russia's Kursk region and doubts over the future of Western aid as President-elect Donald Trump returns to the White House.
On Sunday, Russia staged a massive missile and drone strike on the national power grid that killed seven people and renewed fears over the durability of the hobbled energy network.
'Persistent Russian Attacks'
The embassy in Kyiv urged U.S. 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.
Russian President Vladimir Putin lowered the threshold Tuesday for a nuclear strike in response to a broader range of conventional attacks. Washington said afterwards it had not seen any reason to adjust its nuclear posture.
Commenting on Wednesday's U.S. embassy warning, Andriy Kovalenko, head of the Ukrainian Security Council's Centre for Countering Disinformation, said Russia was ready to conduct more 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.
Kyiv was targeted in an overnight drone attack, which caused minor damage.
The military said it shot down 56 drones and lost track of 58 more as a result of what it described as "active counteraction." Six drones also flew out of Ukraine's air space and two out of six missiles were shot down, it added.
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