After months of political turbulence, Nigeria, Africa's most populous nation, has regained relative stability and swept aside the immediate risk of chaos or even a coup, observers say.

Next year's presidential elections, with all their attendant problems and numerous stakes, are now the next greatest challenge facing Africa's premier oil exporter, they said.

"I would rule out the possibility of a coup now... the country is now a bit stable," said Bayo Okunade, political scientist at Nigeria's premier University of Ibadan.

"We cannot compare where we are now with where we were three months ago. That was a very dark period," he said.

Analysts said that Nigeria, with a history of successive military coups d'etat up till 1999, was spared another one after President Umaru Yar'Adua, suffering from an acute heart ailment, was hospitalised late last year for more than 90 days in Saudi Arabia.

After two and a half months of political vacuum, the parliament on February 9 voted Yar'Adua's deputy, Goodluck Jonathan, into office as acting president, a post that was confirmed when Yar'Adua returned to Nigeria on February 24.

Yar'Adua has neither been seen in public since November nor is his state of health known.

Jonathan, who has assumed power since February, has now appointed his own advisers and ministers and was this week guest of US President Barack Obama in Washington, in his maiden foreign visit.

The probability of a coup "has increased lately," according to an expert on Nigeria at the French Research Institute for Development, Marc-Antoine Perouse de Montclos.

And even though the risk has "diminished" in recent weeks, "it can not be ruled out" he said.

Ishola Williams, a retired army major general, disagreed.

"The army is one of the most ethnically balanced institutions. Every single ethnic group is in the military. So, to have a consensus to make a coup d'etat, the situation would need to be very bad. We haven't got to such a situation now," he told AFP.

"The military wouldn't attempt anything right now," said Williams, former head of Transparency International in Nigeria.

"There is some political stability now... The test will come as we move towards the elections," he added.

Jonathan should demonstrate his capacity to put in place the long-awaited electoral reforms ahead of the 2011 presidential poll and tackle Nigeria's national cankerworm, corruption, Williams said.

Jonathan's ability to consolidate a fragile peace in the oil-rich southern Niger Delta, where Yar'Adua had offered an amnesty to militants, would also be critical.

Violence resurged recently in the key region just as the country was wracked by sectarian clashes which claimed hundreds of lives in the central part of the west African nation.

Under an unwritten rule adopted by the Nigeria's main political party on power rotation between the mainly Muslim north and Christian-dominated south, the next elected president in 2011 should be a northerner.

The political plans of Jonathan, a southern Christian, are yet unknown, and his candidature cannot be ruled out.

However, if he vies for office it "would be destabilising", said Jonas Horner, analyst with the think-tank Eurasia Group in New York.

A good sign that the world's eighth oil exporter appears to be out of the political woods of recent months is that even if Yar'Adua were to pass on now, a much feared possibility for some time, political stability would be less threatened as the transition is well established, observers said.