"We cannot see him [Yar'Adua], we don't know where he is and don't even know that he is doing," Rotimi Akeredolu, the NBA president, said.

"All we are asking is that the vice president be sworn-in as acting president."

The country's constitution states that the president should write to both chambers of the Nigerian parliament if he is unable to rule. His office said that he had left for Saudi Arabia on November 23 last year, but did not formally notify parliament.

'Stronger leader'

Alistair Soyode, the President of the European arm of the Nigerians in Diaspora organisation, told Al Jazeera: "According to the presidency his treatment has improved tremendously. Recently the president has been able to speak to people in Nigeria, including the vice-president, the senate leader and the speaker of the national house of assembly.

"So right now the latest is that the president is getting more treatment and will likely be back on his way of Nigeria soon.

"This is coming officially from one of his special advisors."

Soyode said that he did not believe that Yar'Adua would come back a weaker leader due to his absence, rather the opposite.

Nigeria has recently had the international attention focused upon it after one of its citizens attempted to blow-up an aircraft en route to the US city of Detroit on Christmas Day.

Umar Farouk Abdulmutallab, 23, boarded the airliner in Lagos, Nigeria's largest city, allegedly carrying a highly explosive chemical before flying via Amsterdam to Detroit.

He was arrested by US authorities following the failed attempt to detonate the device in the air.

Yar'Adua has not spoken on the incident, although other senior government leaders have, including Jonathan.