GEOFF BENNETT: Welcome to the "NewsHour."

Water is pouring through a giant gap in a dam in Southern Ukraine tonight.

It gave way early this morning, forcing thousands of people to flee and touching off competing claims about the cause.

AMNA NAWAZ: Ukraine accused the Russians of blowing up the dam.

Moscow charged that Ukrainian shelling caused the rupture.

Neither side offered any direct evidence, but the results were catastrophic.

The massive structure that sustained Ukraine's largest reservoir now overtaken by a thick wall of water.

Satellite images show the ruptured Kakhovka hydroelectric power plant and the violent stream it unleashed.

The torrent burst through the dam, down the Dnipro River and toward the Black Sea.

Flooding threatened tens of thousands of residents of low-lying villages in the water's path.

Ukrainian police surveyed the Kherson region by boat, at times wading through thigh-deep water to carry people to safety.

LIDIA ZUBOVA, Kherson Resident (through translator): Our local school and stadium downtown were flooded.

The stadium was completely underwater and the floodwaters were reaching the school.

The road was completely flooded.

The bus was stuck.

Only one elevated point could be reached by the bus, and this is where we were taken from.

AMNA NAWAZ: In occupied Nova Kakhovka, Russian officials said water levels had risen past 30 feet with more than 600 homes submerged.

The Soviet-era dam was captured by Russian forces at the start of the war and is surrounded by major flash points on the front lines.

It spans the Dnipro River, separating Ukrainian forces from the Russian-occupied south, including Crimea.

To the Northeast, the Zaporizhzhia nuclear power plant sit's along the Kakhovka Reservoir, and just downstream, Kherson, the port city retaken by Ukraine last fall, which has seen some of the war's most intense fighting.

Both sides were quick to blame the other for the breach.

Moscow said it was Ukrainian explosives.

SERGEI SHOIGU, Russian Defense Minister (through translator): Aiming to prevent the offensive operations by the Russian army on this section of the front line, the Kyiv regime committed an act of sabotage.

AMNA NAWAZ: Ukrainian President Volodymyr Zelenskyy responded.

VOLODYMYR ZELENSKYY, Ukrainian President (through translator): It is physically impossible to blow it up from the outside by shelling.

It was mined by the Russian occupiers.

They blew it up.

And this once again demonstrates the cynicism with which Russia treats the people whose land it has captured.

AMNA NAWAZ: At the White House today, U.S. officials said they are still assessing who is responsible.

JOHN KIRBY, NSC Coordinator For Strategic Communications: We are working with the Ukrainians to gather more information, but we cannot say conclusively what happened at this point.

What is clear and what we absolutely can say is that the damage to the Ukrainian people and to the region will be significant.

AMNA NAWAZ: Some reports say Russian mismanagement of the facility led to the incident.

In recent days, high water levels suggest the reservoir could have been perilously overfilled, causing the dam collapse.

Whatever the cause, experts warn of a potential humanitarian and environmental crisis.

The dam provides electricity and drinking water to a huge swathe of Southern Ukraine.

And, in Zaporizhzhia the nuclear power plant relies on the reservoir to cool its systems.

The head of the International Atomic Energy Agency said its six reactors are in shutdown mode, leaving enough water to supply the plant for several months.

While not presently in danger, he warned any further harm to the reservoir could be catastrophic.

RAFAEL GROSSI, Director General, IAEA: It is vital this cooling pond remains intact.

Nothing must be done to potentially undermine its integrity.

I call on all sides to ensure nothing is done to undermine that.

The consequences may be grave.

AMNA NAWAZ: For more on the destruction of the dam and its impact on Ukraine and on the war, we turn to Michael Kofman, senior fellow for Russian studies at the Center for Naval Analyses.

That's a federally funded research and analysis organization that focuses primarily on national security issues.

Michael, as we just saw there, the breach has certainly impacted both Russian-controlled areas and Ukrainian-held areas.

As U.S. officials work to see if they can assess who is responsible, how do you see it?

Would this breach benefit either side?

MICHAEL KOFMAN, Center for Naval Analyses: Now, unfortunately, this breach is an ecological and a humanitarian disaster.

It actually results on a lose-lose proposition for both sides.

The flooding is going to substantially affect the Ukrainian-controlled right side of the riverbank.

It's going to probably damage even more the currently Russian-occupied side of Kherson, because it's on a lower floodplain.

And it will also potentially damage the water canal that supplies water all the way down to Russian-occupied Crimea.

So this is a disaster on many levels, and it will have long-lasting economic and humanitarian implications for the region.

I think, unfortunately, it's still unclear whether the dam was destroyed resulting from a deliberate act by Russian forces or due to negligence, given this dam was previously damaged during the Russian withdrawal from Kherson back in November.

And there was significant rain.

And there was a raising of the water level in the deeper water reservoir ahead of this over the course of the past several months.

So that situation is still unclear.

But, either way, Russia is responsible, either by virtue of action or by virtue of the fact that it controlled the dam.

At the end of the day, the outcome of the situation is the result of Russian actions in Ukraine one way or another.

AMNA NAWAZ: So, as the blame game continues to play out here, what is the military impact?

You mentioned, obviously, downstream there Crimea being impacted.

That's a Russian-occupied area.

Could the loss of that water supply impact the Russian occupation there?

MICHAEL KOFMAN: So, the impact there's likely to be long term and is still unclear.

I think, unfortunately, many of the models projecting what could happen in the situation that were done back in the fall didn't even anticipate this high of a water level coming through the dam.

That being said, I don't think this will substantially affect Ukrainian military prospects when it comes to their offensive this summer.

I think, when you look on a situation along the Dnipro River, on the one hand, the flooding is going to damage the defenses that the Russian military built alongside the riverbank.

On the other hand, it's going to make a Ukrainian cross-river operation exceedingly difficult.

And it was always a precarious proposition or at least a higher risk or low-likelihood proposition to begin with.

So, I don't think it's going to substantially shape outcomes when it comes to Ukraine's offensive operation, which seems to have begun over the course of at least the last two days.

AMNA NAWAZ: What about that Ukrainian counteroffensive?

As you just indicated, there seems to be consensus around the idea that it has, in fact, begun.

It seems like you agree with that.

But what does this mean for both the direction and for the duration of the war?

MICHAEL KOFMAN: So, I think it's fair to say that the Ukrainian offensive has begun and we're probably in initial phases.

I think we're beyond what people consider to be shaping operations or other activities to set the conditions that were taking place over the past several weeks.

There's been a significant uptick in fighting.

There was a limited Ukrainian offensive that has made some headway in one part of the region, Southern Donetsk between Vuhledar and (INAUDIBLE).

So there does seem to be a significant increase in fighting.

That said, the Ukraine offensive is likely to proceed over the course of several months, not days and weeks.

And I don't think we're seeing the main effort or the forces that were likely to be involved in the main effort yet.

So, unfortunately, it's too early to make any predictions or assessment in this regard, except that Ukraine is taking the initiative.

And I'm deeply skeptical that what's happened with this dam and the resultant flooding is actually going to affect their military prospects.

AMNA NAWAZ: Michael, in the minute or so we have left, on the long-term impacts of this flooding, you're talking about tens of thousands of people who have now been deeply impacted who are being evacuated, the disaster on top of the 16 months of war that Russia has been waging there.

What will this now require from the international community that is so invested in supporting Ukraine?

MICHAEL KOFMAN: I think it's going to require a tremendous amount of assistance, especially once Ukraine is able to liberate the territory in Kherson, because that's the territory that will be substantially flooded on the left side of the riverbank, the eastern part of the country.

And it's going to lead to lasting damage to agriculture, provision of drinkable water -- water.

And it's going to wipe out entire communities.

It honestly depends.

We will have to see what happens when the water levels to some extent recede.

But I think there will be lasting damage from this incident.

AMNA NAWAZ: Disaster on top of disaster.

That is Michael Kofman, senior fellow for Russian studies at the Center for Naval Analyses, joining us tonight.

Michael, thank you.