• Footage verified by NBC News showed water cascading over the damaged dam, with evacuation efforts underway to protect thousands of people living in low-lying areas downstream.
• The dam is situated on the Dnieper River and holds back an enormous body of water. It was built in 1956 as part of the Kakhovka hydroelectric power plant.
• Ukraine's nuclear operator, Energoatom, said via Telegram that the water level in the Kakhovka Reservoir "is rapidly decreasing, which is an additional threat" to the Zaporizhzhia nuclear power plant.

A strategically important dam in Russian-occupied southern Ukraine was blown up on Tuesday, raising fears about safety, water supply and Europe's largest nuclear power plant, which receives cooling water from the reservoir upstream.

Ukraine accused Russian forces of blowing up the dam in the Russian-controlled town of Nova Kakhovka. Moscow denies the attack and says Kyiv intentionally sabotaged the dam to distract attention from its counteroffensive, according to Kremlin spokesperson Dmitry Peskov, in Google-translated comments reported by Russian state news agency Tass.

CNBC has not been able to independently verify the claims.

Footage verified by NBC News showed water cascading over the damaged dam, with evacuation efforts underway to protect thousands of people living in low-lying areas downstream.

About 600 homes have been flooded as a result of the Nova Kakhovka dam blast, Russian emergency services said, according to Tass. Moscow declared a state of emergency in the Nova Kakhovka district.

As of 1 p.m. local time (6 a.m. ET), Ukraine's state emergency service said about 1,000 people had been evacuated by rescue services.

The dam is situated on the Dnieper River and holds back an enormous body of water. It was built in 1956 as part of the Kakhovka hydroelectric power plant.

The Kakhovka Reservoir supplies water to the Crimean Peninsula, which was annexed by Russia in 2014, and provides cooling water to the Zaporizhzhia nuclear power plant, which is also under Russian control.

In addition, its water is used to supply hydroelectric stations, irrigation systems and freshwater fish farms.

Ukrainian President Volodymyr Zelenskyy said the government and public services were doing all they could to save people, with those living in so-called danger zones ordered to be evacuated as soon as possible.

Earlier on Tuesday, Zelenskyy convened an emergency meeting of the National Security and Defense Council to discuss the situation.

"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," Zelenskyy said.

'A new sign of escalation'

Ukraine's state-owned hydropower company said the blast that severely damaged the Nova Kakhovka dam came from inside the engine room.

"As a result of detonation of the engine room from the inside, the Kakhovskaya [hydroelectric power plant] was completely destroyed. The station cannot be restored," Ukrhydroenergo said in a statement.

Half the span of the Nova Kakhovka dam appears to have been destroyed and Tass reported the structure is continuing to collapse, citing emergency services.

Meanwhile, a Russian-installed official in the Ukrainian town of Nova Kakhovka reportedly said in comments to state television that the dam would be impossible to repair.

The U.K., Estonia and Lithuania condemned the act as a "war crime," while Latvia warned the blast is an "ecological disaster."

The European Commission, the EU's executive arm, said the destruction of the Kakhovka dam reflected "a new sign of escalation" in Russia's war against Ukraine.

"The European Union condemns strongly this horrific and barbaric attack against such a crucial piece of infrastructure with terrible humanitarian and environmental consequences," said Peter Stano, EU foreign policy spokesperson.

"It is obvious that this is happening in the context of Russia's aggression against Ukraine," he added.

NATO Secretary-General Jens Stoltenberg, meanwhile, said the destruction of the Kakhovka dam endangers the lives of thousands of civilians "and causes severe environmental damage."

"This is an outrageous act, which demonstrates once again the brutality of #Russia's war in #Ukraine," he said via Twitter.

'No immediate risk' to Zaporizhzhia nuclear plant

The U.N.'s nuclear watchdog said it was closely monitoring the situation at the Zaporizhzhia nuclear power plant. The International Atomic Energy Agency said it is aware of the reported damage at the dam, but there was no "immediate safety risk" at the plant.

IAEA Director-General Rafael Grossi said that an absence of cooling water for an extended period at the Zaporizhzhia nuclear plant would cause fuel melt and the inoperability of the emergency diesel generators.

"However, our current assessment is that there is no immediate risk to the safety of the plant," Grossi said in a statement. He added there are a number of alternative sources of water for the nuclear plant.

Ukraine's nuclear operator Energoatom said via Telegram that the water level in the Kakhovka Reservoir "is rapidly decreasing, which is an additional threat" to the nuclear power plant, according to a translation.

"Water from the Kakhovka Reservoir is necessary for the station to receive power for turbine capacitors and safety systems of the ZNPP. The station's cooling pond is now full: as of 8:00 a.m., the water level is 16.6 meters, which is sufficient for the station's needs," the agency said.

The Conflict and Environment Observatory, a nonprofit based in the U.K., said the Kakhovka Reservoir is known to be the "ultimate heat sink" for the Zaporizhzhia nuclear power plant, noting that it can remove heat from shutdown reactors and spent fuel pools. It warned the breach of the dam posed a "serious threat" to the plant.

German Chancellor Olaf Scholz said his government was also monitoring the status of the Zaporizhzhia nuclear power plant following the dam's breach.

Correction: As of 11 a.m. local time (4 a.m. ET), 885 citizens in Ukraine's Kherson region were evacuated, according to the Internal Affairs Ministry. An earlier version misstated a time.