The United States has temporarily shut down its embassy in Kyiv and told staff to "shelter in place" after received what it called "specific information" about an "significant air attack".
The warning was posted on the embassy's website - and minutes later, reports surfaced of explosions in the Ukrainian capital in an indication that Vladimir Putin is turning up the heat.
The Kremlin is fuming at US President Joe Biden's decision to allow Ukraine permission to begin using long-range ATACMS missiles inside Russia.
The statement said: "The US Embassy in Kyiv has received specific information of a potential significant air attack on November 20.
"Out of an abundance of caution, the Embassy will be closed, and Embassy employees are being instructed to shelter in place.
"The US Embassy recommends US citizens be prepared to immediately shelter in the event an air alert is announced."
US citizens are further being urged to monitor local media for updates, identify shelter locations in advance of any air alerts, Immediately take shelter if an air alert is announced, and follow the directions of Ukrainian officials and first responders in the event of an emergency.
One Telegram channel shared the announcement, along with the claim that there had been "explosions again" in the city.
Ukraine fired several American-supplied ATACMS missiles, with a range of roughly 200 miles, into Russia, officials said yesterday, marking the first time Kyiv used the weapons that way in 1,000 days of war.
The use of the weapons came as Russian President Vladimir Putin formally lowered the threshold for using nuclear weapons, opening the door to a potential nuclear response by Moscow to even a conventional attack by any nation supported by a nuclear power. That could in theory include Ukrainian attacks backed by the US
A Telegram channel affiliated with the Ukrainian military posted a video Tuesday that it said shows US-supplied ATACMS missiles being fired from an undisclosed location in Ukraine.
According to a US official, Ukraine fired about eight of the missiles, and just two were intercepted by the Russians.
The official said that the US was still assessing battle damage, but that the missiles struck an ammunition supply location in Karachev, a city of about 18,000 people in Russia's Bryansk region. The official spoke on the condition of anonymity to discuss intelligence assessments.
The developments marked a worrying escalation in the conflict that has repeatedly ratcheted up international tensions. U.S. officials recently expressed dismay at Russia's deployment of North Korean troops to help it fight Ukraine, while Moscow seethed when Washington eased restrictions on the ATACMS in recent days.
The 1,000-day mark has magnified scrutiny of how the war is unfolding and how it might end, amid signs that a turning point may be coming with US President-elect Donald Trump entering the White House in about two months' time. Trump has pledged to swiftly end the war and has criticised the amount the U.S. has spent on supporting Ukraine.
Neither Russia nor Ukraine can sustain the war for a long time, analysts say, though Russia would be able to keep going for longer due to its vaster resources.
Ukraine's forces are under severe Russian pressure on the battlefield at places on the about 1,000-kilometre (600-mile) front line where its army is stretched thin. Ukrainian civilians, meanwhile, have repeatedly been attacked by Russian drones and missiles.
Ukraine's Ministry of Defence claims 1,690 Russian personnel were lost yesterday.
Asked at a press conference in Rio de Janeiro whether Britons should prepare for nuclear war, the Prime Minister said: "This is irresponsible rhetoric coming from Russia and that is not going to deter our support for Ukraine.
"We're now on day 1,000 of a conflict, that's 1,000 days of Russian aggression, 1,000 days of sacrifices in Ukraine.
"We have stood with Ukraine from the start.
"I've been doubling down in my clear message that we need to ensure Ukraine has what is needed for as long as needed to win this war against Putin."
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