More than 100 foreigners have died from various diseases and unknown causes in the past two years in Malaysia's immigration detention centres, according to documents reviewed by the Reuters news agency.

The reported toll, which has not been previously disclosed, is based on Malaysian immigration department data provided to the National Human Rights Commission, which is known by its Malay acronym Suhakam.

Reuters reported on Thursday that documents from the government-funded commission detailed 83 deaths in 2015 and at least 35 in 2016 up to December 20.

More than half of the 118 dead were reportedly from Myanmar, the source for tens of thousands of refugees coming to Malaysia, including Rohingya Muslims escaping persecution by Myanmar's authorities and its majority Buddhist population.

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It is unclear whether the death rate is higher than in neighbouring countries. Government officials in Indonesia and Thailand told Reuters they do not disclose such numbers.

The rate is higher than in countries such as the United States, which in the last financial year recorded 10 deaths in its much larger immigration detention system.

'Appalling'

Malaysian Prime Minister Najib Razak has been a harsh critic of the Myanmar government and its de-facto leader Aung San Suu Kyi after a crackdown in October by Myanmar’s security forces led many Rohingya to flee across its borders amid multiple allegations of mass killings and gang rapes by troops.

He has called for foreign intervention to stop the "genocide" in Myanmar.

Najib's office did not return calls seeking comment.

"The numbers are too many and are shocking and it calls for the overhaul of the system," said Jerald Joseph, one of eight commissioners at Suhakam, which is due to publicly announce the numbers next week in its annual report on human rights issues in Malaysia.

Joseph described conditions at the centres, some of which he had visited, as "appalling" and said the deaths should be investigated as a criminal matter.

The illnesses that led to some of the deaths may have been caused or exacerbated by poor sanitation and food, physical abuse and a lack of medical attention, said Joseph, who was speaking on behalf of the commission.

Malaysia’s home ministry, which oversees the immigration department, said it was trying to improve the conditions in the centres but that its budget was constrained.

"I agree there is some overcrowding and the conditions are not ideal. We are always trying to improve the procedures, health conditions and management of these sites. The problem is we hit a budget brick wall," deputy home minister Nur Jazlan Mohamed told Reuters.