The United States' envoy to Sudan has warned that el-Fasher, a besieged city in western Darfur, could fall to rebel forces imminently.

El-Fasher is the only city still under army control in the western Darfur region.

The armed forces have been fighting the paramilitary Rapid Support Forces (RSF) in a civil war that has lasted for 14 months.

US envoy Tom Perriello told the BBC that some in the RSF think capturing El-Fasher will help them establish Darfur as a breakaway state.

Mr Perriello said the US would not recognise an independent Darfur "under any circumstances".

"I think if there's anyone in RSF territory who thinks taking el-Fasher means somehow they will have a right to the state of Darfur, they need to disillusion themselves of that myth," he said. "It does not mean that.

He also called for a ceasefire in the city, which the RSF has been attacking since the middle of April.

"We see upwards of a million innocent people being starved by the siege of the RSF," he said. "Bombings have killed people inside of hospitals.

"We see 45,000 pregnant women who not only have no real prenatal care, but don't even have enough meals a day to be nourished enough for a healthy pregnancy.

"And as bad as it is, it could get worse any day if el-Fasher falls, not only the horrors that would come from the battle, but as people flee."

America's warning about the fall of el-Fasher follows weeks of bloody fighting in the city.

Hundreds of thousands of civilians are trapped inside the town, with many enduring hunger and thirst amidst shortages of food and water.

El-Fasher had been a sanctuary for many who had fled their homes because of the conflict. But it has now turned into yet another frontline.

Civilians there have reported being hit by shelling and bullets in their homes and even in hospital.

One of the last health facilities still running is the Sayyid Shuada health centre.

Footage filmed in the hospital for the BBC shows a distraught mother too shocked to speak after shelling hit her home, injuring five family members.

She held her toddler as her husband and other children received emergency treatment.

The mother said she couldn't tell whether the blood on her toddler's face was his, his siblings’ or his father’s.

Sayyid Shuada is overstretched by the mounting casualties.

“Every day there is a new wave of patients that are wounded arriving... on average maybe 50 per day, which is already what we consider mass casualty," said Claire Nicolet, who leads medical charity MSF's emergency response in Sudan.

There's only one surgeon present at the facility who is forced by the situation to work "round the clock".

"Most of [the patients] need surgery so it's pretty dramatic," says Ms Nicolet.

Local community volunteers have formed committees to try and support the hospital. They take care of non-medical tasks, like finding water and fuel and collecting data.

Volunteer Khalid Abdul Hamid tells the BBC committees are collecting donations of cash, goods and services, including from the already war-battered community.

“From our own efforts and the efforts of well-wishers, we have managed to get some medicine… or cash contributions to buy medicine from the local market," he said.

The situation is deteriorating by the day and an increasing number of facilities are put out of operation by the fighting.

On Saturday, RSF fighters stormed the South Hospital, a referral hospital that was treating civilians wounded in the war. Gunmen opened fire and looted the facility, stealing an ambulance.

The hospital, which was also ran by the medical charity MSF, has now been closed.

Its head of emergencies, Michel Lacharite, said the attack was outrageous. “Opening fire inside a hospital crosses a line," he said.

The South hospital had been hit by shelling and bullets at least three times in 10 days before the Saturday raid.

A paediatric hospital managed by the MSF in el-Fasher was bombed in May, killing two children.

The constant shelling in th city has sent tens of thousands fleeing once again. Most are heading towards Sudan's west, with options for safer places running out.

"We need this to end," Mr Perriello said.

"We need cooler heads to prevail and get this particular battle paused, while also we don't take our eye off of other parts of Sudan."