AMNA NAWAZ: More than a year of brutal war in Sudan between Sudan's army and a rebel paramilitary force has killed tens of thousands of people and forced more than eight million to flee their homes.

Some five million are now at risk of starvation.

As Nick Schifrin reports, there is new evidence of ethnic cleansing -- Nick.

NICK SCHIFRIN: That's right, Amna.

This week, Human Rights Watch provided that evidence of not only ethnic cleansing, but also war crimes and crimes against humanity against the paramilitary Rapid Support Forces in Western Sudan's Darfur.

It's there that the RSF and this new war have produced the world's largest displacement and hunger crises.

And now new evidence reveals the sheer scale of atrocities faced by civilians, with hundreds of thousands still at risk.

In Darfur's El Geneina, they lived to tell the tale of what happened here, of history being repeated.

Beginning last summer, local activists bore witness to what Human Rights Watch and the U.S. government label ethnic cleansing.

They recorded how their city of half-a-million was burned.

Where there used to be homes and schools that became shelters for the displaced, there is now only ash.

The targets were members of the Massalit tribe, many women and children, who suffered looting, rape, and torture, the ethnic violence unleashed by the paramilitary Rapid Support Forces, or RSF, and other Arab militias.

The RSF descends from government-backed Janjaweed militias that, in the 2000s, brutally crushed an uprising and killed hundreds of thousands of non-Arabs.

The U.S. labeled it genocide.

These 2023 videos obtained and reported by the "NewsHour" last summer are part of a new Human Rights Watch report released this week, "The Massalit Will Not Come Home."

Tens of thousands of them have been forced to flee and are now refugees who fill camps in neighboring Chad.

Human Rights Watch says this is what the RSF's violence was designed to do, drive them from their homes permanently.

TOM PERRIELLO, U.S. Special Envoy for Sudan: For the past year, the people of Sudan have suffered slaughter, crimes against humanity, sexual violence, ethnic cleansing.

NICK SCHIFRIN: Tom Perriello is the U.S. special envoy for Sudan.

He and the U.N. are now warning of a new RSF assault on El Fasher, home to Sudan's largest camp for the displaced.

Doctors Without Borders says some 15,000 children there are suffering from acute malnutrition.

TOM PERRIELLO: The RSF and its leadership are rooted in the Janjaweed militias who committed genocide 20 years ago when I first went to Darfur.

And, in particular, if we see the fall of El Fasher, we will see a new level of suffering for this war.

NICK SCHIFRIN: The other side of this civil war, which has turned residential streets into battlegrounds, is the Sudanese Armed Forces, or SAF, which the U.S. has also accused of war crimes.

This week, USAID Administrator Samantha Power accused the Sudanese Armed Forces of -- quote -- "obstructing" humanitarian aid from reaching Darfur, and she said nearly 900 thousand of Darfur's most vulnerable had received no assistance at all since early April, ahead of what she called looming famine.

To discuss the situation in Darfur and Human Rights Watch's new report, I'm joined by Tirana Hassan, Human Rights Watch's executive director.

Tirana Hassan, thank you very much.

Welcome to the "NewsHour."

You guys collected 221 witness statements from the worst-hit parts of Darfur.

What did those witnesses tell you?

TIRANA HASSAN, Executive Director, Human Rights Watch: We have received and documented grueling testimony from victims and survivors, eyewitnesses of the attacks on predominantly the Massalit and non-Arab populations in El Geneina.

What we document, as you noted in your report, reportage, was that there's most definitely been all indications that the ethnic cleansing is taking place.

The violence and the targeted killings and attacks on the Massalit and other non-Arab communities has been -- shows all the hallmarks of ethnic cleansing.

But we have also documented war crimes and crimes against humanity that have been committed by the Rapid Support Forces, as well as in concert with allied militia across West Darfur, and particularly in El Geneina.

So this campaign has been absolutely brutal.

Our researchers were on -- have been on the ground and speaking to people inside Sudan for the last 10 months, and we have documented a comprehensive account of the large-scale atrocities that have taken place.

We are talking about houses and entire neighborhoods being burnt to the ground and looted.

We have documented thousands of people that have been killed, but we really don't know what the true death toll is.

It could be much, much larger.

And, as you noted earlier, half-a-million people are now driven from Darfur into Chad by the violence.

And even on their journeys, they told us that they were -- continued to be hunted down, targeted and attacked, women raped on the journey to safety in Chad.

NICK SCHIFRIN: As I noted, the Rapid Support Forces, the RSF, descends from the Janjaweed militias, which in the 2000s were accused of genocide.

Do you believe, does Human Rights Watch believe that it is possible genocide is happening again?

TIRANA HASSAN: Well, what we have been able to conclude from our current research is that there are most definitely war crimes and crimes against humanity have been committed.

We also believe that there is the possibility that some of the acts that we have documented were genocidal in nature.

And what that really means is, there needs to be an investigation and a collection of evidence so that an international court can actually gather evidence to be able to see if the crime of genocide has taken place.

This isn't just, though, about if it has taken place.

We are now at a point where these same forces, the Rapid Support Forces and their allied militias, are currently surrounding the town of El Fasher.

So we should be talking also about genocide prevention.

We know we are on the cusp of mass atrocities if the international community doesn't start to mobilize to ensure that there is some sort of force, some sort of mechanism to ensure that there is civilian protection.

What we have seen in El Geneina is a very grim snapshot and indicates what could happen and the atrocities that could unfold in El Fasher in the coming days or weeks.

NICK SCHIFRIN: You just said some kind of mechanism.

What does the international community need to do, what does the United States need to do to try and help prevent further atrocities, especially El Fasher, which, as we noted, is the location of the largest collection of internally displaced within Darfur.

TIRANA HASSAN: That's right.

I mean, the population of El Fasher is approximately 1.5 million people, if not more.

So the level of urgency that we need to see from the United Nations and the African Union is that they should urgently deploy a mission to Sudan to protect civilians, and they should start in locations where the most are at risk of these deliberate attacks.

But on top of that, we need to track back and look at, where are the weapons coming from?

There are arms embargoes already, but we know that they are in place for Darfur.

They need to be expanded to all of Sudan, because, outside of the Darfur region, we also know that there are possible war crimes being committed across the country.

And one of the reasons we are seeing this sort of level of just uncontrolled violence and the Rapid Support Forces committing these atrocities town after town is because there is a history of no accountability.

So, ensuring that the International Criminal Court is resourced enough, well enough to be able to conduct the investigations and ensure that there is just as an accountability for the crimes committed is not only important for the survivors for today's atrocities, but it also sends a message to the -- to those responsible for the war -- for war crimes and crimes against humanity that this -- that there is no more impunity.

These times are done.

NICK SCHIFRIN: And, finally, in the short time we have left, it's important to note the Sudanese Armed Forces, as I mentioned before, Samantha Power, others in the U.S. have called them out for blocking the aid that would go from Chad to Darfur.

The Sudanese Armed Forces wants to be seen as the legitimate rulers of Sudan.

What are you calling on the Sudanese Armed Forces to do?

TIRANA HASSAN: We are calling for the Sudanese Armed Forces to also -- who also have been implicated in possible war crimes, to abide by international law, to protect civilians.

I mean, one of the things that we have seen is that they have been deploying munitions with large explosive capacities in densely populated areas, which we know has led to civilian casualties.

But, on top of that, in a conflict, it is -- there are rules even in wars.

And the Sudanese Armed Forces, as well as the Rapid Support Forces, need to be ensuring that there is full, free, unfettered humanitarian aid.

People are already living through such horrors.

To add no access to water, no access to food and basic shelter just compounds the inhumanity that the population is currently suffering through.

NICK SCHIFRIN: Tirana Hassan, executive director of Human Rights Watch, thank you very much.

TIRANA HASSAN: Thank you for having me.