A Nigerian police unit that focuses on robberies was accused by Amnesty International on Wednesday of torturing suspects held in custody and demanding bribes to free them.

The human rights campaign group said people arrested by the Special Anti-Robbery Squad (SARS) were subjected to hanging, starvation, beatings, shootings and mock executions "until they either make a 'confession' or pay officers a bribe to be released".

A Nigeria Police Force spokesman did not respond to calls and text message requests for a comment on Amnesty's report.

A 32-year-old man, Chidi Oluchi, told Amnesty he was tortured after being arrested by SARS officers in the southeastern city of Enugu.

"They started beating me with the side of their machetes and heavy sticks. My mouth was bleeding and my vision became blurred," he said, adding that he was released after paying SARS officers 25,500 naira ($80).

Amnesty said it was told by a senior officer that around 40 officers accused of manhandling detainees had been transferred to other stations in April 2016, although he did not say whether the claims against them had been investigated.

"It is time for the authorities to ensure that officers responsible for such human rights violations are finally held accountable," said Amnesty's Nigeria researcher, Damian Ugwu.

"There is also an urgent need for robust legislation that ensures all acts of torture are offences under Nigeria's criminal law."