CAIRO/TINE, Chad, Dec 3 - The Sudanese paramilitary force that besieged then overran a city in Darfur in late October is systematically holding trapped residents for ransom, killing or beating those whose families cannot pay, witnesses, aid workers and researchers say. 

Reuters could not determine exactly how many people the paramilitary Rapid Support Forces (RSF) and allied militias have detained in and around al-Fashir, capital of North Darfur. But the accounts suggest that large groups are being held in a cluster of villages within 80 km (50 miles) of al-Fashir, while others have been brought back into the city as the RSF demands payments worth thousands of dollars from their relatives. 

Their detention shows the risks faced by those who were unable to reach safety from al-Fashir, which had been the final significant holdout against the RSF in the western Darfur region before its fall. Witnesses have described mass reprisals including summary executions and sexual violence since the RSF takeover.

It also sheds light on the plight of some of the tens of thousands of people who are unaccounted for as aid agencies push to gain access to famine-stricken al-Fashir and its environs, which became a focal point in the 2 1/2-year-old war between the RSF and Sudan's army.

Reuters interviewed 33 former captives as well as 10 aid workers and researchers, who provided previously unreported details about the violence captives faced, the locations where they were held and the scale of the detentions. 

Survivors described paying ransoms of between 5 million ($1,400) and 60 million ($17,000) Sudanese pounds - vast sums in an impoverished region. 

Many of those who could not pay were shot at close range or mowed down in groups, 11 survivors said, while other captives were badly beaten. A Reuters reporter saw survivors who had fled over the border to Chad bearing injuries that appeared to be from beatings and gunshots. Reuters could not verify their accounts in full.

"They give you three or four days, and if you don't transfer the money, they kill you," said Mohamed Ismail, who spoke with Reuters...