WILLIAM BRANGHAM: Sudan's civil war has entered a new, horrific phase.

The paramilitary Rapid Support Forces have gone on a killing rampage after taking over the key city of El Fasher in Western Darfur after over a year-and-a-half of siege.

Hundreds of thousands have fled to neighboring Tawila, escaping famine and mass executions.

Nick Schifrin has this report.

But a warning: Images and accounts in this story are disturbing.

NICK SCHIFRIN: This week, the people of El Fasher, beaten and threatened, attacked and hunted, fled for their lives from a murderous militia that films itself unleashing ferocious violence.

A fighter shows off his work.

"We have burned them," he says.

"We have burned them."

They show off their horror and document their own war crimes with videos too graphic to show.

The paramilitary Rapid Support Forces turned an El Fasher convoy trying to escape into a killing field, a ditch into a mass grave, and a home for healing into a death trap.

The U.N.

and international humanitarian groups accuse the RSF of entering El Fasher's Saudi Maternity Hospital and killing everyone inside.

At least 450 people are dead.

Survivor Fatma Abdulrahman tries to smile, but is burdened by unimaginable loss and trauma.

FATMA ABDULRAHMAN, Displaced from El Fasher (through translator): Shelling killed my daughter, injured my other daughter's eye and paralyzed my son.

I was hit by shelling.

My body is full of wounds.

NICK SCHIFRIN: El Fasher was the last holdout in Darfur of the Sudanese Armed Forces, which has fought the RSF in a brutal three-year civil war.

Sudanese cities have become battlegrounds.

Both sides are accused of war crimes.

But, in Darfur, the RSF is accused of genocide.

The rebels descend from government-backed Janjaweed militias that in the 2000s brutally crushed and uprising and killed hundreds of thousands of non-Arab.

Then too the U.S.

labeled their actions genocide.

For the last 18 months, the RSF surrounded El Fasher, forcing residents to starve.

It was a famine inside a siege that became a slaughter.

Many fled here, Tawila, the closest city to El Fasher.

Ikram Abdelhamed's daughter and son-in-law were both killed, and so she is now raising her 2-month-old grandson with almost no food.

For two weeks, she's fed him only moldy grain and rehydration salts, surrounded by a family that is shocked and shattered, as she told a local journalist.

IKRAM ABDELHAMED, Family Member of Victims (through translator): We came running.

They were chasing us, firing rockets over our heads.

They took the men out and lined them up, and they shot them in front of us.

They lined them up and shot them in the street and left them.

NICK SCHIFRIN: Another man fled alongside 200 people.

Only four survived, as he told the U.N.

's Population Fund anonymously.

MAN (through translator): My head was on the ground, and I could see to my right two women, my neighbors lying dead, blood flowing from their heads.

Then the RSF got into their vehicles and started running us over.

After they left, those of us who survived continued walking.

Another RSF group caught us and demanded a $2,800 ransom from each of us.

Only four of us managed to pay.

The rest were killed.

They killed children, the elderly, and women.

NICK SCHIFRIN: Inside El Fasher, the RSF left behind pools of blood large enough to be seen from space, massacres across the city, as documented by Yale's Humanitarian Research Lab.

And the killing was targeted.

The RSF murdered activist and former lawmaker Siham Hassan Hasaballah, the youngest person ever elected to Sudan's Parliament, back in her hometown of El Fasher helping the hungry.

And Mohammed Eldouda, who was the spokesperson for the nearby Zamzam refugee camp, he filmed these scenes of a people facing famine.

MOHAMMED ELDOUDA, Slain Spokesperson, Zamzam Camp: We are now in acute starving and acute hunger.

NICK SCHIFRIN: He was murdered this week for transmitting the truth.

SYLVAIN PENICAUD, Doctors Without Borders: It's some kind of blind violence.

It's not only about taking the city, but it really looks now like some kind of revenge.

NICK SCHIFRIN: Sylvain Penicaud is the project coordinator in Tawila for Doctors Without Borders.

He and MSF doctors are treating a population that's fled to Tawila after being targeted for their ethnicity.

SYLVAIN PENICAUD: The civilians are being massively targeted.

They are not so-called collateral damages, but they are being targeted as people living in El Fasher and people who dare to resist.

Women, are on an extreme level of pain, extremely exposed to sexual violence.

NICK SCHIFRIN: And in Tawila, children arrive on the verge of starvation, and some of them orphaned.

SYLVAIN PENICAUD: There are children that are seeing probably their parents die under their very eyes.

So, yes, this war, as any war, is quite dirty and will have consequences for years and years.

That's for sure.

NICK SCHIFRIN: International aid organizations have been warning about an attack on El Fasher for more than a year and say its residents were let down by the international community.

For more, I turn to Anmar Homeida, the executive director of the Sudanese American Physicians Association, or SAPA, which is still operating in Tawila near El Fasher.

Dr.

Anmar, thanks very much.

Welcome to the "News Hour."

As I just said, your organization is still operating in Tawila and you were operating just until this week in El Fasher itself.

Do you know what's happened to your teams there?

DR.

ANMAR HOMEIDA, Executive Director, Sudanese American Physicians Association: It's really devastating the amount of violence that happened against people, against civilians, against women and children, against even like humanitarian actors and medical providers.

Current, we have them in our guesthouse in Tawila.

The amount of killing specifically against, like, humanitarian actors and medical providers is quite painful for all of us to hear.

NICK SCHIFRIN: And what is the state of people who have fled El Fasher to arrive in Tawila?

DR.

ANMAR HOMEIDA: Most of them are women and children.

They don't have any place to go to.

Right now, the shelters are very limited.

Even the basic dignifying services, they are not able to get it.

Adding to that, the different kind of trauma.

And the most sad part right now, they don't even know the situation of whom they left behind inside El Fasher yet.

NICK SCHIFRIN: As I mentioned in the package, El Fasher was the last stronghold of the Sudanese Armed Forces, which, of course, is the organization running the country that's been fighting the RSF.

What is the impact of that on Darfur and in fact all of Sudan?

DR.

ANMAR HOMEIDA: This war started in the center of the capital of Sudan, Khartoum, which left most of the health care system destroyed.

And more than 14 million people have been displaced, adding to that also more than 25 million people on the verge of food insecurity.

So the disastrous consequences of this war are quite heinous.

But Darfur specifically suffer more because of the continuous complex situation that is currently happening there since like the early 2000s.

So, the basic infrastructure is fully destroyed, including access to safe shelter or basic health needs, specifically for women and children.

It's kind of, like, disastrous.

Yet the local actors are still trying to access to food, basic nutrition services and, of course, secure health care interventions and safe water and shelter.

NICK SCHIFRIN: Dr.

Anmar Homeida, thank you very much.

DR.

ANMAR HOMEIDA: Thank you.

Thank you for having me.