World powers Thursday waded into the row in Nigeria over the president's health, lamenting the "uncertainty" caused by his absence as his deputy promised that he would return home soon.

"Nigeria is going through a period of uncertainty because of the prolonged illness of President Musa Umaru Yar'Adua," said a joint statement by the United States, European Union, Britain and France.

The powers said they welcomed constitutional efforts in Nigeria, Africa's most populous nation and one of the world's top oil producers, to "resolve the question of governing authority in the president's prolonged absence".

The 58-year-old president has been receiving treatment for a serious heart condition in Saudi Arabia since November 23.

"Nigeria's stability and democracy carry great significance beyond its immediate borders," given its importance as a global oil producer and stabilising force in west Africa, said the statement.

It was signed by US Secretary of State Hillary Clinton, British Foreign Secretary David Miliband, France's Foreign Minister Bernard Kouchner and European Union High Representative Catherine Ashton.

Angola Press

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