CNN  — 

A major dam and hydro-electric power plant in Russian-occupied southern Ukraine suffered a collapse early Tuesday, prompting mass evacuations and fears for large-scale devastation as Ukraine accused Moscow’s forces of committing an act of “ecocide.”

Residents downstream from the Nova Kakhovka dam on the Dnipro River in Kherson were told to “do everything you can to save your life,” according to the head of Ukraine’s Kherson region military administration, as video showed a deluge of water gushing from a huge breach in the dam.

The critical Nova Kakhovka dam is the largest reservoir in Ukraine in terms of volume. It’s the last of the cascade of six Soviet-era dams on the Dnipro River, a major waterway running through southeastern Ukraine. There are multiple towns and cities downstream, including Kherson, a city of some 300,000 people before Moscow’s invasion of its neighbor.

Here is what we know about the crisis.

What happened?

It is unclear what caused the dam to collapse in the late evening of Monday or early hours of Tuesday.

A CNN analysis of satellite imagery from Maxar shows the dam was damaged just days before suffering the structural collapse.

The satellite images show the road bridge that ran across the dam was intact on May 28. However, imagery from June 5 shows a section of the same bridge missing. Analysis of lower-resolution satellite imagery suggests the loss of the bridge section took place between June 1 and 2.

CNN cannot independently verify whether the damage to the road bridge played a part in the dam’s collapse, or whether it was destroyed in a deliberate attack by one of the warring parties.

Both Ukrainian and Russian officials said the dam collapsed in an explosion and are blaming each other for it. The incident happened as Ukraine was gearing up for a widely anticipated counter-offensive.

The Ukrainian military intelligence said an explosion occurred at 2:50 a.m. local time on Tuesday (7.50 p.m. ET Monday), when “Russian terrorists carried out an internal explosion of the structures of the Kakhovka hydro-electric power plant.”

Meanwhile, the Russian-installed mayor of Nova Kakhovka, Vladimir Leontiev, initially denied the dam had collapsed in an interview with Russian state media RIA Novosti, calling it “nonsense.” He later confirmed the destruction of parts of the dam in what he called “a serious terrorist act” but said there was “no need to evacuate.”

CNN was not immediately able to verify the claims made by Ukrainian and Russian officials.

The Kremlin on Tuesday rejected the accusations. In his regular call with journalists, Kremlin spokesperson Dmitry Peskov claimed the attack was “planned and carried out by order received from Kyiv, from the Kyiv regime,” aiming to “deprive Crimea of water.”

United Nations Secretary General António Guterres he believed it was clear that the dam destruction in Kherson region is “another devastating consequence of the Russian invasion of Ukraine,” but added that the UN doesn’t have access to information to independently verify the cause.

What are the consequences?

The dam holds back around 18 cubic kilometers of water in the Kakhovka Reservoir, about equal to the Great Salt Lake in the US state of Utah.

Mohammad Heidarzadeh, senior lecturer in Department of Architecture and Civil Engineering at the University of Bath in England, said the Kakhovka reservoir is one of the largest dams in the world in terms of capacity.

“It is obvious that the failure of this dam will definitely have extensive long-term ecological and environmental negative consequences not only for Ukraine but for neighbouring countries and regions,” Heidarzadeh told Science Media Centre on Tuesday, adding that the Kakhovka dam was an “embankment” dam, which means it was made of gravel and rock with a clay core in the middle.

“These types of dams are extremely vulnerable, and are usually washed away quickly in case of a partial breach… a partial damage is sufficient to cause a complete collapse of the dam because water flow can easily wash away the soil materials of the dam body in just a few hours,” he added.