At least 13 people have been killed after an attack on one of the last remaining hospitals in el-Fasher, a Sudanese city trapped under siege.

Sixteen others, including a doctor and nurse, were injured after the paramilitary Rapid Support Forces (RSF) shelled Saudi hospital several times on Tuesday night, a source there told the BBC. A group of Sudanese medics has called the attack a war crime.

Pictures showed shattered windows, cracks from shrapnel, a gaping hole in the mud-brick wall and twisted metal from hospital beds covering the floor.

The RSF has been besieging el-Fasher for more than 17 months, leaving hundreds of thousands of people stuck in the city, facing starvation.

The paramilitary group is fighting the army for full control of el-Fasher, the last military stronghold in the vast Darfur region.

This is the second strike on the Saudi hospital this year - the first in January killed three children and injured three others.

The latest shelling ripped through part of the hospital, destroying wards.

In recent weeks, the RSF has intensified its assault on el-Fasher, leading experts to believe the city could soon fall unless the army receives immediate reinforcements.

The two sides have been engaged in a ferocious civil war for more than two years, causing the world's worst humanitarian crisis and tens of thousands of deaths.

On Tuesday research showed that the RSF has finished building an earthen wall around el-Fasher, bolstering their siege and making it even harder for civilians to escape.

The RSF began building the berm - or raised bank - in May, satellite imagery analysed by Yale University's Humanitarian Research Lab (HRL) found.

All major exit routes are now sealed by the 57-kilometre (35-mile) wall and civilians attempting to flee have reported extortion, arbitrary detentions, disappearances, and sexual violence at RSF-controlled checkpoints.

The incessant fighting in el-Fasher has forced most health facilities to shut. Aid convoys carrying food and healthcare have been blocked from reaching civilians.

"After over 500 days of unremitting siege by the RSF and incessant fighting, El Fasher is on the precipice of an even greater catastrophe if urgent measures are not taken [to] loosen the armed vice upon the city and to protect civilians," UN High Commissioner for Human Rights Volker Türk said last Thursday.