After decades of military rule, coups and strong-armed elections, Africa's most populous nation now finds itself with a seriously ill, and absent, president.

For weeks, President Umaru Yar'Adua has been hospitalized in Saudi Arabia for what staffers say is a serious heart condition. With no clear successor, Nigeria is roiled by uncertainty and some prominent citizens are even calling for his resignation. Even militants in the oil-rich Niger Delta, whom Yar'Adua brought into peace talks only weeks ago, now worry they have no "good faith partner" to negotiate an end to attacks that have cut into Nigeria's oil-dependent economy.

The president, long troubled by a kidney ailment and poor health, did not formally appoint an acting leader in the West African country before he left, as the constitution requires. The constitution puts Vice President Goodluck Jonathan next in line, but it's unclear if the Muslim-dominated north would allow the Muslim Yar'Adua to be replaced with a Christian, as Jonathan is. Customarily, the Nigerian presidency alternates between Christian and Muslim leaders. Northerner Yar'Adua still has two years left in his term.

As Yar'Adua's absence grew from days to weeks, a group of 50 prominent Nigerians this month issued a petition calling on Yar'Adua to resign if he's medically incapable of running the country.

The vice president has sat in on federal meetings in Yar'Adua's place. Dora Akunyili, Nigeria's information minister, said Friday that she cannot comment on the president's absence or the growing calls for him to step down from office.

"I don't think anything has changed," Akunyili said, adding that the president is responding well to treatment. Further pressed, she asked a reporter to call her back "after Christmas."

But negotiations with militants in the Delta remain at a sensitive point. Their attacks on the infrastructure and kidnappings of oil workers cut Nigeria's oil production by about a million barrels a day, allowing Angola to surge ahead as Africa's top oil producer.

The Movement for the Emancipation of the Niger Delta, the main militant group in the region, held formal peace talks with Yar'Adua only two weeks because he left for Saudi Arabia. The group had declared an unconditional cease-fire Oct. 25 and attacks dropped off in the region.

Jomo Gbomo, a MEND spokesman, issued a statement Friday night to The Associated Press saying that if Yar'Adua left office, the government "will have no other choice than to continue his policies."