YAOUNDE, Cameroon, Aug. 6 -- Cameroonian President Paul Biya has been out of the country for two months, fueling concerns over his health and criticism from opposition leaders over what they describe as a growing leadership vacuum in the Central African nation.

Biya, 93, the world's oldest serving president, has ruled Cameroon since 1982. He began his eighth term in November after winning a disputed presidential election in October. He left Cameroon on June 7 for what the presidency described as a "brief private stay in Europe," but the trip has become one of his longest overseas absences. The government later confirmed that he is in Switzerland.

The veteran leader's health has long been the subject of speculation as he frequently spend...