Ill President Umaru Yar'Adua returned home Wednesday after a three-month stay at a Saudi Arabian hospital, but left his second-in-command in control of the oil-rich country's fragile democracy.

A statement Wednesday from presidential spokesman Olusegun Adeniyi said Goodluck Jonathan would continue to serve as acting president.

That decision appears to protect Jonathan's position, as Nigeria's lawmakers used extraconstitutional means to put him into power two weeks ago. It also raises questions about Yar'Adua's health, as shaky television footage showed an ambulance leaving the presidential wing of the capital airport but offered no images of a leader who hasn't been seen publicly since leaving Nigeria on Nov. 23.

"President Yar'Adua wishes to reassure all Nigerians that on account of their unceasing prayers and by the special grace of God, his health has greatly improved," the statement read. "However, while the president completes his recuperation, Vice President Jonathan will continue to oversee the affairs of state."

Yar'Adua's statement offered no other details about his health.

Abdullah Aminchi, Nigeria's ambassador to Saudi Arabia, said Yar'Adua needs time to recuperate before assuming presidential powers.

"He's already walking. He eats. He can move about," the ambassador said. "I think he just needs some time to rest and he can go back to his work as president of Nigeria."

Reporters at the presidential villa saw Yar'Adua's chief aide, Col. Onoyveta Mustapha, before a scheduled meeting of the Cabinet on Wednesday but caught no glimpse of the president.

Mustapha had been one of only a few trusted aides who stayed with and had access to Yar'Adua while he received treatment at the King Faisal Specialist Hospital and Research Center in Jeddah. Those surrounding Yar'Adua blocked a delegation of lawmakers from visiting him earlier this month.

Yar'Adua was admitted into the hospital in Saudi Arabia the day after he left Nigeria. As questions mounted, his physician released a statement saying Yar'Adua suffered from acute pericarditis, an inflammation of the sac surrounding the heart.

Though the country's constitution calls for the president to offer a written letter empowering the vice president to take over in his absence, Yar'Adua did not follow that procedure. The Nigerian government ground to a halt in Yar'Adua's absence as oil contracts went unsigned and people in the streets worried about the future of Africa's most populous nation.

After more than two months of a standstill, the National Assembly voted to empower Vice President Goodluck Jonathan to take over as acting president. However, the parliament's action specified that Jonathan had to cede power to Yar'Adua upon his return if he's medically capable of leading the nation of 150 million. The lawmakers' vote also went beyond the process envisioned by the constitution, presenting questions on how exactly Jonathan would transfer power back to Yar'Adua.

Yar'Adua long has been plagued by poor health and kidney ailments. During his 2007 presidential campaign, he left the country two weeks before the vote to receive medical care in Germany after experiencing what he described as a shortness of breath. His absence sparked enough concern then that outgoing President Olusegun Obasanjo even made a telephone call to Yar'Adua during a political rally to ask his candidate: "Umaru, are you dead?"

Despite those health concerns, Yar'Adua became president through an election marred by fraud, intimidation and violence. It marked the first time power was transferred from one elected civilian to another in Nigeria, which became independent from Britain in 1960.

News of Yar'Adua's return received a quick response from the U.S., where Secretary of State Hillary Rodham Clinton has issued calls to Nigeria to respect and follow its constitution. In a statement Wednesday, Assistant Secretary of State Johnnie Carson said the U.S. welcomed Yar'Adua's return, but still had concerns about the nation's future.

"Recent reports ... continue to suggest that President Yar'Adua's health remains fragile and that he may still be unable to fulfill the demands of his office," Carson said. "We hope that President Yar'Adua's return to Nigeria is not an effort by his senior advisers to upset Nigeria's stability and create renewed uncertainty in the democratic process."