Human Rights Watch has criticised "life-threatening" conditions in Egyptian prisons, saying it has independently recorded nine deaths in custody since mid-2013.

The New York-based group said in a press release published on Wednesday that detainees seem to have died "after being tortured or physically abused."

It also documented that many died due to "overcrowded cells" or lack of "adequate medical care for serious ailments."

"Yet the authorities have taken no serious steps either to improve detention conditions or to independently investigate detainees’ deaths," it said.

HRW said that such conditions violate the Egyptian constitution.

Commenting on the report, interior ministry spokesman Hani Abdel-Latif told Reuters on Wednesday that "such talk has no basis in truth."

"This is very strange talk and hasn't been said anywhere else before," he added.

In its report, HRW cites other fatality figures documented by Egyptian groups, including the Nadeem Center for the Rehabilitation of Victims of Violence, which found at least 35 people dead in custody, mostly in police stations, between early June and early September 2014.   

Nadeem identified a cause of death in 15 of these cases, out of which 13 involved overcrowding or lack of medical care, while two involved physical abuse.

HRW has also criticised the increasing number of arrests and lengthy preventative detentions in Egypt, particularly after the passing of a controversial protest law that has seen hundreds arrested nationwide.

In the press release, it narrated the case of three people who died in prison from beating, lack of life-saving medical care and continued detention despite terminal illness.