Dos Santos's influential daughter Isabel this week moved to deny rumours the president had died in Spain.
Dos Santos's influential daughter Isabel this week moved to deny rumours the president had died in Spain, where he was confirmed to have travelled by state media on May 2.
"We are all concerned about the lack of official information on the health of the president," Rafael Savimbi, a senior figure in UNITA, the main opposition party, told AFP.
Agostinho dos Santos, a political analyst close to the opposition, said "serious and clear explanations" were needed after reports of the president's health troubles and rumours of his death.
Opposition news website Maka Angola said that the president was "between life and death in a private clinic in Barcelona."
President dos Santos has been in power since 1979 and has announced that he will not contest elections due in August, marking a historic change in the oil-rich country.
Isabel dos Santos, a billionaire businesswoman, this week criticised rumours about her father's health.
"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," she said on her Instagram account.
The president oversaw the end of the country's civil war in 2002, but has been regularly accused of crushing dissent and allowing his family and associates to enrich themselves as most Angolans endure extreme poverty.
Joao Lourenco, the current defence minister, has emerged as Dos Santos' chosen successor and is on course to take power after the elections.
The MPLA party has ruled since independence from Portugal in 1975, and authorities suppress opposition activity.
"We must remain calm and not disseminate irresponsible information," Bento Kangamba dos Santos, a senior member of the MPLA, told local media.
Dos Santos has reportedly received cancer treatment in Barcelona over several years.
Late last year the MPLA issued a statement denying reports that he was ill and criticising social media messages.
The president has been rarely seen in public but exercises authority over all branches of government, politics, media and business.