Ukraine on Tuesday accused Russian forces of blowing up a major dam and hydroelectric power station in a part of southern Ukraine that Russia controls, sending water gushing from the breached facility and risking massive flooding. Ukrainian authorities ordered hundreds of thousands of residents downriver to evacuate.

Russian officials countered that the Kakhovka dam was damaged by Ukrainian military strikes in the contested area.

The fallout could have broad consequences: Flooding homes, streets and businesses downstream; depleting water levels upstream that help cool Europe’s largest nuclear power plant; and draining supplies of drinking water to the south in Crimea, which Russia has illegally annexed.

The dam break added a complex new element to Russia’s ongoing war in Ukraine, now in its 16th month, as Ukrainian forces were widely seen to be moving forward with a long-anticipated counteroffensive in patches along more than 620 miles of frontline in the east and south of Ukraine.

Ukraine’s nuclear operator Energoatom said in a Telegram statement that the blowing up of the dam “could have negative consequences for the Zaporizhzhia nuclear power plant,” near the dam, but at the moment the situation is “controllable.”

According to the Ukraine War Environmental Consequences Working Group, a total collapse in the dam would wash away much of the left bank and a severe drop in the reservoir has the potential to deprive the nuclear plant of crucial cooling, as well as dry up the water supply in northern Crimea.

A senior advisor to President Zelensky, Mykhailo Podolyak,  said that “a global ecological disaster is playing out now, online, and thousands of animals and ecosystems will be destroyed in the next few hours.”

Videos posted online began testifying to the spillover: One showed floodwaters inundating a long roadway, another showed a beaver scurrying for high ground from rising waters.

Mr. Zelensky called an emergency meeting to deal with the crisis, Ukrainian officials said.

The Ukrainian Interior Ministry called for residents of 10 villages on the river’s right bank and parts of the city of Kherson downriver to gather essential documents and pets, turn off appliances, and leave, while cautioning against possible disinformation.

The Russian-installed mayor of Nova Kakhovka, Vladimir Leontyev, said the strikes were “a very serious terrorist act” said Moscow-appointed authorities are “preparing for the worst consequences” — though stopping short of urging an evacuation of city residents.

Ukraine controls five of the six dams along the Dnipro, which runs from its northern border with Belarus down to the Black Sea and is crucial for the entire country’s drinking water and power supply.

Footage from what appeared to be a monitoring camera overlooking the dam that was circulating on social media purported to show a flash, explosion and breakage of the dam.

The head of the Kherson Regional Military Administration, Oleksandr Prokudin, said in an early morning video posted to Telegram that “the Russian army has committed yet another act of terror,” and warned that water will reach “critical levels” within hours.

The Kakhovska dam was completely destroyed, Ukraine’s state hydro power generating company wrote in a statement: “The station cannot be restored.” Ukrhydroenergo also claimed that Russia blew up the station from inside the engine room.

The Russian-appointed mayor, Mr. Leontyev, said Tuesday that numerous strikes on the Kakhovka hydroelectric plant destroyed its valves, and “water from the Kakhovka reservoir began to uncontrollably flow downstream.” 

He added that damage to the station was beyond repair, and it would have to be rebuilt. 

Ukraine and Russia have previously accused each other of targeting the dam with attacks, and last October Mr. Zelensky predicted that Russia would destroy the dam in order to cause a flood.