Yar'Adua's return last week threw the country into confusion and revived concerns of a leadership squabble just two weeks after his vice president Jonathan was installed as acting president after a nearly three-month leadership vacuum.

"What is important first and foremost... is who is the recognized leader of Nigeria, and right now I believe it is acting president Goodluck Jonathan," State Department spokesman Philip Crowley told reporters when asked who Washington recognizes.

"We are concerned that his return is not an effort by his senior advisers to upset Nigeria's stability in an effort to forward their own personal interests," Crowley said.

"Nigeria at this point needs a strong and effective leader to ensure the stability of the country" and manage its economic, political and security challenges," Crowley said.

Yar'Adua returned Wednesday after a three-month stay in a Saudi hospital fueling concern about fresh turmoil, but his deputy said all was "on course."