Some 200 people were reported killed in clashes Sunday triggered by predawn attacks by Muslims against three Christian villages near the flash-point regional capital of Jos, scene of earlier religious violence.
The bloodshed, the latest sectarian eruption in recent weeks, underscored the increasing fragility of the continent's most populous country as it approaches presidential elections in 2011 amid a power struggle that is essentially between the Muslim-dominated north and the largely Christian south.
Nigeria's crisis was triggered in late November when ailing President Umaru Ya'Adua, a northerner, was flown to Saudi Arabia for treatment of a purported heart problem. But he failed to delegate presidential power to his deputy, Goodluck Jonathan, a southern Christian.
That triggered a constitutional crisis that paralyzed government because no one appeared to be in charge.
A fragile truce in the southern oil zone of the Niger River Delta between Yar'Adua and tribal insurgents, who had badly crippled the all-important oil industry for five years, began to fall apart in the political vacuum.
Parliament tried to find a solution by conferring presidential power on Jonathan in Yar'Adua's absence. But northerner political barons balked at that as they feared it signaled a Christian takeover of the presidency.
Under an unwritten agreement, the north holds the presidency for two terms -- 10 years -- and then it rotates to a southerner. That arrangement, introduced after military rule ended in 1999, was threatened by Yar'Adua's lengthy absence.
After three months in Saudi Arabia, he made a secrecy-shrouded return to the capital Feb. 24, apparently to block what was seen as a southern move to install Jonathan as head of state before Yar'Adua had finished his first term.
Last Wednesday, the ruling People's Democratic Party said it wants a northern Muslim to stand as its candidate next year. That would cut out Jonathan, the acting president, from making a run for election.
Yar'Adua remains too ill to govern and reportedly is being treated in a mobile intensive care unit. He hasn't been seen in public, while chieftains from north and south wrestle for power in a political environment that is notorious for its tribal intrigues and feuds, secret deals and betrayals.
Control of the oil industry is a key element of power in Nigeria because it lubricates a vast patronage network and feeds the rampant corruption that infects the state machine at every level.
The country has a history of military coups but the army chief of staff, Lt. Gen. Abdulrahman Dambazau, a northern Muslim, declared last week that "nobody, no matter what … will drag us into it.
"This is a difficult period for everyone but we know it's a political thing," he told a gathering of retired officers. "We're not politicians. We know our constitutional responsibility … and we'll stick to it."
However, when Yar'Adua's aircraft landed at Abuja before dawn Feb. 24, he was whisked away to the presidential compound by the army without Jonathan's knowledge and remains under military guard.
The flare-up of sectarian tensions in the so-called Middle Belt where ethnic groups mingle in a swathe of fertile and hotly contested land between the Muslim north and Christian south could deepen the political crisis.
The oil industry, which provides 90 percent of Nigeria's revenue, is also at risk if the southern insurgency flares again.
Before fighting halted with an amnesty declared by Yar'Adua in August, later consolidated by a rebel cease-fire, Nigeria's oil production had been slashed by one-third, to around 1.5 million barrels per day.
Nigeria is the United States' fifth largest oil supplier.
Renewed hostilities could cause even more damage, heightening problems in the oil industry as the government seeks to introduce reforms that would give the state a larger return from oil exports than it gets from its current agreement with foreign oil companies that operate the fields.
Jos has a history of communal violence, in which thousands of Muslim and Christians have perished since 2001. More than 320 people were killed in clashes in January by police count.