Ukraine has accused Russian forces of blowing up a major dam and hydroelectric power station in a part of southern Ukraine that Russia controls.
Reports on social media say 11 of the 28 spans of the dam have been destroyed, allowing millions of gallons of water to flow into the Dnipro River.
Russian military bloggers say the dam was blown at around 2am but, like much of the information being released, it cannot be verified, leaving many to question who, in fact, is responsible for the attack.
Zelensky blames 'Russian terrorists'
The Ukrainians blame the Russians and that is the most plausible explanation because this will make any assault across the Dnipro much more difficult as the Ukrainian counter-offensive progresses because this is one of only two crossing points south of Zaporizhia city.
The dam is about 20 miles upstream from the city of Kherson, so it's a key crossing point of the river.
On social media, President Volodymyr Zelensky called the attack on the dam an act of terrorism by the Kremlin. Back in October, the Ukrainian president had, in fact, warned the European Council that the Russians had mined the dam.
Mr Zelensky said on Twitter: "Russian terrorists. The destruction of the Kakhovka hydroelectric power plant dam only confirms for the whole world that they must be expelled from every corner of Ukrainian land.
"Not a single meter should be left to them, because they use every meter for terror. It's only Ukraine's victory that will return security. And this victory will come. The terrorists will not be able to stop Ukraine with water, missiles or anything else.
He added: "All services are working. I have convened the National Security and Defense Council. Please spread official and verified information only."
The Russians occupying the south of the Dnipro blame the Ukrainians, saying they shelled it and Russia's Interfax news agency says it was down to structural failure, which does have a little more credibility because the Russians have allowed the reservoir to fill to record levels.
In turn, by blowing the dam up, the Russians would also be affecting the water supply to Crimea, which for now they occupy.
So there is a military, strategic angle to this, but this reservoir also provides water to help cool the reactors at the Zaporizhia nuclear power plant, the largest in Europe.
There are concerns that the damage to the dam could have broad consequences: flooded homes, streets and businesses downstream; depleted water levels upstream that help cool Europe's largest nuclear power station; and drained supplies of drinking water to the south in Crimea, which Russia illegally annexed.
At the moment, four of the six reactors are shut down and the International Atomic Energy Agency says the plant's not in danger currently.
Thousands of people are being evacuated, with wildlife in danger too, while Mr Zelensky said he convened an urgent meeting of the National Security Council.
He alleged that Russian forces set off a blast inside the dam structure and that some 80 settlements were in danger.
An 'abhorrent act' - says UK Foreign Secretary
Foreign Secretary James Cleverly called the damage to the major dam a "catastrophe" and an "abhorrent act".
Mr Cleverly tweeted: "The destruction of Kakhovka dam is an abhorrent act," the Foreign Secretary said. "Intentionally attacking exclusively civilian infrastructure is a war crime.
"The UK stands ready to support Ukraine and those affected by this catastrophe."
The Foreign Secretary recently met with Volodymyr Zelensky to discuss how best the UK can continue to support Ukraine, on and off the battlefield.
The UK is said to be offering additional support to Ukraine following the destruction of the dam.
The Prime Minister's spokesman said: "We are ready to offer humanitarian and economic support.
"With regards to the dam itself, I think it's too early to say specifically if we are providing support.
"We have provided things like generators and other equipment to Ukraine previously."
Downing Street has said that it "wouldn't rule out" raising an attack on a major dam in Ukraine with Russia.
Asked whether there are plans to bring up the issue with Russia, Prime Minister Rishi Sunak's official spokesman told reporters: "I wouldn't rule that out at this stage.
"I think obviously at the moment we are looking into the situation and if there is more to update on later on, we will do so."
Asked whether the Government believes Russia blew up the Kakhovka dam, the official said: "We are carefully monitoring the situation and we stand ready to support those affected.
"As the Foreign Secretary said, none of this would be taking place if it was not for Russia's illegal invasion. We are steadfast in our support."