Also on Thursday, Olusegun Obasanjo, Nigeria's former president, suggested that his ailing successor should step aside.
'Path of honour'
"If you take up a job, elected ...and then your health starts to fail you and you will not able to deliver, to satisfy yourself and satisfy the people you are supposed to serve, then there is a path of honour and path of morality," Obasanjo said.
It was the ex-president's first public statement on Yar'Adua's illness.
Obasanjo handpicked Yar'Adua to run for elections after his attempts for a third term failed.
He said he knew that Yar'Adua had a history of kidney problems, but doctors had found him fit, denying suggestions that he had handpicked an unfit leader.
"I asked experts who then told me that if you were on dialysis and you are no longer on dialysis, it means you have had a successful kidney transplant," he said.
Yar'Adua was taken ill on November 23 and flown to Saudi Arabia for specialised treatment. The president has spent nearly 60 days in Jeddah for treatment for acute pericarditis, an inflammation of the membrane covering the heart.
The opposition and critics claim Yar'Adua's prolonged absence and lack of information on his condition have stalled government business and caused a power vacuum.