The Rapid Support Forces (RSF) militia reportedly killed hundreds of civilians at the main hospital in el-Fasher, days after it captured the Sudanese city, the head of the UN's health agency says.

Tedros Adhanom Ghebreyesus said the UN health agency was "appalled and deeply shocked" by the reported killing of 460 people at the hospital.

Earlier, the Sudan Doctors' Network said that on Tuesday, RSF fighters had "cold-bloodedly killed everyone they found inside the Saudi Hospital, including patients, their companions, and anyone else present".

It gave no casualty figures, but said medical facilities in the city had been "transformed into human slaughterhouses".

The Sudan Doctors Network has also accused the RSF of kidnapping six medics - including four doctors, a pharmacist and a nurse - and reportedly demanding ransoms in excess of $150,000 (£114,000) for their release.

Tuesday's attack on the Saudi Hospital was also reported by the el-Fasher Resistance Committee, a group of local activists, which said there was "a horrifying silence" afterwards.

The city had been the army's last stronghold in the Darfur region, and was captured by the RSF on Sunday after an 18-month siege marked by starvation and heavy bombardment.

Since the conflict erupted in April 2023, the RSF and allied Arab militia in Darfur have been accused of targeting people from non-Arab ethnic groups - allegations the RSF denies.

With the fall of el-Fasher, the UN, activists and aid agencies have expressed fear over the fate of the estimated 250,000 people trapped in the city, many from non-Arab communities.

A communications blackout has made it difficult to confirm what is happening.

BBC Verify has analysed new videos posted to social media showing RSF fighters executing a number of unarmed people in the last few days.

With the difficulties in getting reports from the ground, aid agencies say the full scale of the devastation in and around el-Fasher is only beginning to emerge.

Some people have managed to make the dangerous journey to the town of Tawila, about 60km (37 miles) west of el-Fasher, and described the extreme violence they faced.

"The shelling was so intense on Saturday that we had no choice but to flee el-Fashir," one man told BBC Arabic's Sudan Lifeline programme.

"Along the way, the RSF filmed us and we were beaten and insulted - and they stole what we had on the journey. A number of people were captured and ransoms were demanded for their release.

"Some of those who were taken were later executed. During the journey, many people were arrested, and we suffered greatly from hunger and thirst."

Jan Egeland, a former top UN humanitarian official, told the BBC the situation was catastrophic.

"We have had massacres on top on all of those months of deprivation, starvation, no medical care," he said.

"I think this is the worst place on Earth now; it's the biggest humanitarian emergency on Earth and it happens in the dark, really - there has been far too little attention to what's happening in Sudan."

Dr Tedros said prior to the Saudi Hospital attack, the WHO had verified 185 attacks on health care facilities since the start of the war, resulting in 1,204 deaths.

"All attacks on health care must stop immediately and unconditionally. All patients, health personnel and health facilities must be protected under international humanitarian law. Ceasefire!" he said.

The capture of el-Fasher effectively splits the country, with the RSF now in control of most of Darfur and much of neighbouring Kordofan and the army holding the capital, Khartoum, central and eastern regions along the Red Sea.

The two warring rivals had been allies - coming to power together in a coup in 2021 - but fell out over an internationally backed plan to move towards civilian rule.