Dam supplying water to Crimea blown up in southern Ukraine

This satellite image provided by Maxar Technologies shows an overview of the Kakhovka dam in southern Ukraine on Monday, June 5. AP-Yonhap

Ukraine and Russia accused each other on Tuesday of blowing up a dam and causing widespread flooding in southern Ukraine, while Russia said it had thwarted another Ukrainian offensive in eastern Donetsk and inflicted heavy losses.

Russia launched a new wave of overnight air strikes on Kyiv and Ukraine said its air defence systems downed more than 20 cruise missiles on their approach to the city.

The South command of Ukraine's Armed Forces said Russian forces blew up the Soviet-era Nova Kakhovka dam in occupied Kherson region.

Unverified videos on social media showed intense explosions around the dam and water surging through. The dam, 30 metres (yards) tall and 3.2 km long, was built in 1956 on the Dnipro river.

It holds water equal to that in the Great Salt Lake in the U.S. state of Utah and also supplies water to Crimea, annexed by Russia in 2014, and to the Zaporizhzhia nuclear plant, which is also under Russian control.

"The scale of the destruction, the speed and volumes of water, and the likely areas of inundation are being clarified," the Ukrainian military said on Facebook.

Russian news agencies said the dam had been destroyed in shelling while the mayor of Russia-controlled Nova Kahhovka city was quoted as blaming an act of terrorism - Russian shorthand for an attack by Ukraine.

There was no "critical danger" to the Russian-controlled Zaporizhzhia facility - Europe's largest nuclear plant - Russia's TASS state agency cited a Moscow-backed official in the Zaporizhzhia region as saying.

The Russian installed head of the Kherson region said evacuation near the dam has begun and that water would reach critical levels within five hours.

Reuters could not independently verify the reports and it was unclear whether any of the latest fighting marked the beginning of Ukraine's long-anticipated counter-offensive.

Ukraine's President Volodymyr Zelenskiy will hold an emergency meeting over the Nova Kakhova dam blast in southern Ukraine, Oleksiy Danilov, secretary of Ukraine's National Security and Defence Council, said on Twitter on Tuesday.

Ukrainian officials have made no mention of any broad, significant new campaign, although in his nightly address on Monday, Ukrainian President Volodymyr Zelenskiy was enigmatic, hailing "the news we have been waiting for" and forward moves in Bakhmut in Donetsk.

In this photo provided by the Ukrainian Presidential Press Office, Ukrainian Foreign Minister Dmytro Kuleba, left, and British Foreign Secretary James Cleverly, right, pose for a photo in Kyiv, Ukraine, Monday, June 5. AP-Yonhap

Ukrainian attacks

Russian President Vladimir Putin sent troops into Ukraine on Feb. 24 last year in what the Kremlin expected to be a swift operation, but its forces suffered a series of defeats and regrouped in the country's east.

Tens of thousands of Russian troops dug in over the winter, besieging Bakhmut for months and bracing for an expected Ukrainian counter-attack to try to cut Russia's so-called land bridge to the Crimean Peninsula.

Russia says it thwarted a major Ukrainian attack in the Donetsk region over the weekend and on Tuesday the defence ministry said a fresh Ukrainian assault had also been repelled.

Russian forces inflicted huge personnel losses on attacking Ukrainian forces and destroyed 28 tanks, including eight Leopard main battle tanks and 109 armoured vehicles, it said. Total Ukrainian losses amounted to 1,500 troops.

"Having suffered heavy losses the day before, the Kiev regime reorganized the remnants of the 23rd and 31st mechanized brigades into separate consolidated units, which continued offensive operations," the ministry said on Telegram.

"A complex fire defeat was inflicted by army forces, assault and operational-tactical aviation, missile forces and artillery, as well as heavy flamethrower systems."

There was no immediate comment from Kyiv about Russia's assertions. Russia and Ukraine have often made claims of inflicting heavy human losses on each other which could not be verified.

The Washington Post reported that some U.S. officials thought Ukraine's counter-offensive was underway, but White House national security spokesperson John Kirby declined to comment on whether this was the case.

"I'm not going to be talking for the Ukrainian military," he told a briefing, adding that the United States had done "everything we could ... to make sure that they had all the equipment, the training, the capabilities to be successful."

The success or failure of a counteroffensive, expected to be waged with billions of dollars worth of advanced Western weaponry, is likely to influence the shape of future Western diplomatic and military support for Ukraine.

Ukrainian Foreign Minister Dmytro Kuleba told Reuters on Monday that Ukraine now had enough weapons for a counteroffensive but declined to comment when asked whether it had begun.

In its evening report on Monday, Ukraine's General Staff made no mention of any large-scale offensive, nor did it suggest any deviation from the usual tempo or scope of fighting along front lines that have not changed significantly for months.

Deputy Defence Minister Hanna Maliar said on Telegram that Ukraine was "shifting to offensive actions" along parts of the front but dismissed suggestions of a major operation.

Writing on Telegram, Russia's Wagner militia leader Yevgeny Prigozhin said Moscow's claims of huge Ukrainian losses were "simply wild and absurd science fiction."

Russia now controls at least 18 percent of internationally recognised Ukrainian territory and has claimed four more regions of Ukraine as Russian territory after annexing Crimea in 2014. (Reuters)

Ukrainian soldiers stand in their positions near Bakhmut, background, in the Donetsk region, Ukraine, Monday, June 5. AP-Yonhap