Meles Zenawi, the Ethiopian prime minister, is in a "critical" condition in hospital in Brussels and may not survive, according to diplomatic sources.
Other diplomats told the AFP that Mr Zenawi might not survive his illness.
"He is in a critical state, his life is in danger," said one.
"He is in a critical state but is alive," another added.
Ethiopia's ambassador in Brussels and the hospital authorities refused to comment on the reports.
In Addis Ababa, however, Bereket Simon, a government spokesman, insisted that Mr Zenawi, who has held power in the populous Horn of Africa nation for over two decades, was recovering. "He is not in a critical state. He is in good condition," he told AFP.
Questions surfaced about Mr Meles's health when he missed a two-day African Union summit Sunday and Monday in Addis Ababa, apparently for the first time since 1991. He was last seen looking thin and pale at the G20 summit in Mexico in June.
Whatever Mr Zenawi's condition, anger is growing among Ethiopians at the refusal of his government to provide clarity on the situation and speculation has begun to swirl about possible successors.
The one-time Marxist, who toppled the brutal dictatorship of Mengistu Haile Mariam in 1991, has run Ethiopia through strongly centralised control for two decades and analysts struggle to envisage how the country would be operate without him.
Adjoa Anyimadu, Chatham House's Horn of Africa expert, said that Mr Zenawi's force of personality meant that few other Ethiopian politicians were well-known.
"He is the face of the Ethiopian ruling class so it's difficult to see who would take over from him," she said.
"Ethiopia is also very closely involved in regional issues, from its conflict with Eritrea, mediating between South and North Sudan and providing troops to Somalia. The level of uncertainty if this carries on could have repercussions for the region."