JOHANNESBURG (Reuters) - Angola’s opposition called on the government on Friday to reveal the state of President Jose Eduardo dos Santos’ health amid reports that Africa’s second-longest ruler is seriously ill at a hospital in Spain.

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The 74-year-old dos Santos, who has run the oil-producing southern African nation since 1979, left Angola at the start of May on what was officially billed as a two-week “private visit” but is yet to return.

State media, the only channel through which the government communicates, has maintained total silence on the issue, despite a report on an Angola-related Facebook page last week saying dos Santos had died.

His daughter, Isabel, took to Instagram at the weekend to knock down the report, dismissing it as “fake news”, but did not provide any additional information, fuelling more rumors and speculation.

“Someone has gone so low as to invent information about the death of a man in order to create confusion and turmoil in Angolan politics,” Isabel dos Santos, who is also head of state oil firm Sonangol, said.

Attempts to obtain comment from dos Santos’ spokesman were unsuccessful.

Raul Danda, parliamentary president of the opposition UNITA party, said the lack of clarity about the health of the man who has been central to Angola’s stability since the end of a long civil war in 2002 was becoming a national security issue.

“The health of the President is a matter of concern to everyone, but the problem is that everything about the health of the President is top secret,” he told Reuters in Johannesburg.

Dos Santos, a Soviet-trained oil engineer and veteran of the guerrilla war against Portuguese rule, rarely appears in public but the last time Danda saw him, he did not look well, he said.

“I think that his health is not OK. Knowing him as I do you can easily notice that the man is not OK,” he said. “He was meant to be back on Monday and it’s now Friday and he’s not here.”

The Maka Angola website run by journalist Rafael Marques, a rare dissident in what is one of Africa’s most secretive and repressive states, said dos Santos had suffered a “transient ischemic cerebral accident” - essentially a stroke.

Dos Santos traveled to Barcelona in 2013 for what was widely reported at the time as treatment for prostate cancer.

Angola is due to hold a general election on Aug. 23 that will mark the formal end of dos Santos’ 38 years at the helm of Africa’s number two crude producer and third-largest economy.

His ruling MPLA party, in power since independence in 1975, is almost certain to win a large majority, putting Defence Minister Joao Lourenco, 63, its presidential candidate, into the top job.

However, dos Santos is due to continue as head of the MPLA, leading many Angolans to believe that if his health endures he will remain a powerful force in domestic and regional politics.