Ghana's vice president took the presidential oath of office Tuesday, hours after officials announced the sudden death of President John Atta Mills.

John Dramani Mahama takes over the remaining term of Mr. Mills, who died earlier in the day at a military hospital in the capital, Accra. Mr. Mills was 68.

The president's office said Mr. Mills died a “sudden and untimely death,” a few hours after falling ill. The nature of his illness was not given.

U.S. President Barack Obama paid tribute to Mr. Mills, praising his efforts to improve human rights and the lives of his people.

President Mills had led Ghana since January 2009, after narrowly winning the 2008 presidential election. The election was hailed as a rare example of a peaceful transfer of power in Africa. The new elections are due in December.