Human Rights Watch (HRW) has called on the Tunisian government to conduct an impartial investigation into the suspicious death of two detainees.

The New York-based rights group said on Sunday that the death of Sofien Dridi and Qais Berrhouma while in detention should be probed.

"The Tunisian authorities should carry out a thorough and impartial investigation of these deaths," the group said in a statement.

The HRW says Dridi was found in a hospital morgue in the capital, Tunis, on September 18, one week after he was detained by police at an airport following his deportation from Switzerland.

A prison spokesperson said Dridi, who suffered from diabetes, had died of a heart attack.

Berrhouma was reportedly also arrested by the National Guard anti-drug brigade on October 5, after he was beaten by plain-clothes police in the street. His family was notified that his body was at the same hospital.

"Relatives of both men provided photographs showing marks and bruises on their faces and bodies, and said that authorities had failed to inform them promptly about the deaths," the statement added.

According to lawyers representing the families of the two men, a court in Tunis has launched preliminary inquiries into the deaths.

The rights group says it has also documented three other similar cases of suspicious deaths in custody in Tunisia since 2013, including Abdelmajid Ejday, Mohamed Ali Snoussi, and Walid Denguir.