BANGKOK (AFP) - Thai prisons are massively overcrowded with some inmates forced to sleep sideways, a rights group said Tuesday (Feb 28), urging reform in a country with the world's sixth largest jail population.

The kingdom's worst facilities are five times over their capacity, with inmates forced to sleep on their sides or lay their legs over one another, according to the report by the International Federation for Human Rights (FIDH).

Successive governments have done "very little" to tackle the overcrowding, said FIDH's South-east Asia director Andrea Giorgetta, adding that food, sanitation and medical care are also woefully inadequate.

With 425 prisoners for every 100,000 people, Thailand has the tenth highest incarceration rate in the world and the highest in Southeast Asia.

The booming prison population is largely a result of harsh anti-drug laws from a war on narcotics launched more than a decade ago.

Possession of a few methamphetamine pills - known on the street as yaba - can earn a suspect a decade in jail.

The ruling junta, which seized power in 2014, admits that hardline laws have failed to stem addiction rates.

Rehabilitation for small-time dealers or users is gaining ground as an alternative to jail, although there has yet to be a formal policy shift.

As of January more than 70 percent of Thailand's prisoners were serving time for drug-related offences, according to figures obtained by FIDH.

The proportion is even higher among the 39,000 female prisoners in Thailand, which jails more women per capita than any other nation.

"We are working on it... the structure of our drug laws must be reformed so that offences are decriminalised," the permanent-secretary of Thailand's Justice Ministry, Charnchao Chaiyanukij, told AFP.

Authorities are also seeking to spread the inmate population more evenly across the country's jails, he added, stressing that not all facilities are overcrowded.

The report also noted growing restrictions on access to prisons, stricter visitor regulations and the increased use of secretive military barracks to detain prisoners since the junta came to power.

The junta has outlawed protests since 2014 and detained hundreds of civilians at military bases for expressing political opinions.

Particular alarm has been raised over a detention site in Bangkok's 11th Army Circle, a barracks where two people have died in custody and several have alleged torture.