Mass killings, rape, torture, abductions and forced cannibalism have led to an increase in mental illness in South Sudan, with patients routinely housed in prisons due to an "almost total" absence of mental health care, a rights group said on Wednesday.

There are only two practicing psychiatrists for South Sudan's 11 million people, Amnesty International said in a report ahead of the country's fifth anniversary of independence on July 9.

"My mind is not good," the report quoted one man, Phillip, as saying as he described being forced to eat the flesh of dead men rounded up and shot in a security forces building in the capital, Juba, when conflict broke out in December 2013.