Nigeria's first lady was on Friday sworn into a senior government post in the oil-rich state of Bayelsa, an appointment tagged as scandalous by some opponents.

Patience Jonathan, wife of President Goodluck Jonathan, was named a permanent secretary in Bayelsa by Governor Seriake Dickson.

Dickson is said to be close to the president and accusations of nepotism and patronage have long plagued Nigerian politics, but the governor insisted that the first lady "merits the appointment".

"I have not done anything wrong and illegal," Dickson said at the ceremony.

On Sunday Frank-Oputu, the state chair of the Conference of Nigerian Political Parties, an opposition coalition, said the appointment was greased "with the oil of corruption".

He charged that she lacked the requisite experience and that her official credentials were doctored following the public uproar that ensued when her appointment was revealed on July 11.

"They must have re-arranged the document to cover-up the scandal that followed the announcement," he said.

While Bayelsa is one the top three oil rich states in Nigeria, the world's eight largest producer, the sector is managed federally, with state officials playing almost no formal role in the petroleum industry.

A permanent secretary is the administrative head of a ministry and Jonathan will be assigned a specific portfolio at a later date.