The Associated Press and the Agence France Presse reported that Mrs. Jonathan has been receiving medical treatment for the past 10 days in Germany for food poisoning.
In the first newspaper report of the First Lady's illness, Daily Trust reported on Monday that 55-year-old Mrs. Jonathan took ill and travelled abroad last week.
But a spokesman for the First Lady, Mr. Ayo Osinlu, told Daily Trust on Sunday that she went abroad to "take a moment's rest" and not for medical attention.
AFP yesterday quoted a Presidency source as saying: "It is true that the First lady is ill. She was flown to Germany for medical treatment some 10 days ago. Initially it was for food poisoning that we know of, but we learnt that she underwent surgery last week. I really don't know yet the purpose of the surgery."
The source added that she had been expected to return to Nigeria last week, but "the German hospital is insisting on her full recovery before her discharge."
The Associated Press said a government official told the agency that Mrs. Jonathan fell sick about 10 days ago, following her hosting a summit of First Ladies from across Africa.
As her case of food poisoning worsened, she was flown to Germany for medical treatment, the official said.
The official spoke on condition of anonymity as information about Jonathan's illness had yet to be officially made public, despite numerous newspaper reports and rumors circulating on the internet.
President Goodluck Jonathan has made no public comments about his wife's health.