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.

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.