US human rights groups have said they believe Algerian national Abdul Aziz Naji, who was sent back home from Guantanamo Bay against his will, was being held in secret detention and was at risk of abuse.

"He is at home in Batna," said a judicial source who did not want to be identified, referring to a town about 500 km (310 miles) east of the Algerian capital.

"He just needs to go every week to the local police station to sign a form," said the source.

The Algiers Prosecutor's office said in a statement that Naji been held in detention in Algeria in accordance with legislation on holding terrorism suspects which allows for a person to be held for up to 12 days before appearing in court.

The statement said he was released after appearing before a judge on Sunday who placed him under judicial control — which means he has to report regularly to police pending a further decision on his case.

"Contrary to what has been falsely reported, this person's case has been dealt with in the most complete transparency and in respect for the law, whether in terms of procedure or the length of his detention," the statement said.