LAGOS (AP) -- The ruling party candidate for Nigeria's crucial presidential elections is undergoing medical consultations related to a kidney ailment, party officials said Tuesday.

Katsina Governor Umar Yar'Adua, of the People's Democratic Party, is "with his doctors" because of a kidney complaint, said a party official, who spoke on condition of anonymity citing party rules on dealing with the media. A statement issued by the party said Yar'Adua had left the country. It gave no specifics on his condition but said, "There is no cause for alarm."

Yar'Adua, 56, is a presumptive front-runner in the April poll meant to give Nigeria its first-ever chance at a successful civilian-to-civilian transfer of power.

President Olusegun Obasanjo, who is barred from running by term limits, won 1999 elections that ended years of military rule. He won re-election in a 2003 vote the opposition said was rigged.

With little democratic tradition, Yar'Adua's elimination from the race could cause potentially dangerous jockeying by Nigeria's political elite, many of whom are involved in the country's oil industry.

There have already been political-related killings in the campaign.

Other top candidates in the April 21 vote include Atiku Abubakar, Obasanjo's vice president, and Gen. Muhammadu Buhari, a former military leader.

Nigeria is Africa's most populous nation and the continent's biggest oil producer. But years of corrupt governance have left the country's people among the world's poorest.