An extensive earthen wall is being built around the besieged Sudanese city of el-Fasher and is intended to trap people inside, according to research from Yale University.

From satellite images, the university's Humanitarian Research Lab has identified more than 31km (19 miles) of berms - as the raised banks are known - constructed since May in territory outside the city occupied by the paramilitary Rapid Support Forces (RSF).

El-Fasher, under siege for more than a year, is the last major foothold in Darfur for the army, which has been battling the RSF since April 2023.

The Sudan Doctors Network has told the BBC the RSF is intensifying its offensive there and deliberately targeting civilians.

Yesterday there was a shelling in a civilian area down in the city centre that ended up killing almost 24 civilians and injuring 55 people, among them five women, Dr Mohamed Faisal Hassan, from the medics association, told the BBC's Newsday programme.

The attacks on the central market and a residential area were deliberate and heinous, he said.

Three days ago they targeted one of the biggest hospitals in el-Fasher resulting in a massive massacre of patients and medical staff.

BBC Verify says both sides in the conflict have been using berms as a defensive strategy. But the analysis by Yale's Humanitarian Research Lab (HRL), which has been closely monitoring the conflict, suggests that the RSF is creating a literal kill box around el-Fasher.

The HRL traced the construction of the earthen walls onto a map of el-Fasher. The HRL report notes that the physical boundary deepens siege conditions and control of who and what can enter or leave the city, where about 300,000 people live.

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

Humanitarian organisations have been unable to access el-Fasher for months, and the remaining civilians are enduring constant bombardment, food scarcity, and difficulties in accessing medical care.

Civilians report ongoing attacks when attempting to flee, complicating their situation severely. Adeeb Abdel Rahman Youssef, a former governor of central Darfur State, appealed to the UN Security Council to protect the civilians of el-Fasher, stating, The civilian population in el-Fasher is paying the heaviest price. There is no one to protect them.