Nigerian President Umaru Yar'Adua has returned home after three months' treatment in Saudi Arabia, officials say, but his condition is unknown.A plane from Jeddah landed at the presidential wing of Abuja airport in the middle of the night, where an ambulance was waiting on the tarmac.
Earlier this month, Vice-President Goodluck Jonathan became acting leader as fears mounted of a power vacuum.
Mr Yar'Adua has been getting treatment for heart and kidney problems.
The rest of Wednesday is expected to be crucial, as Nigerians wait to see whether the 58-year-old leader turns up to a scheduled cabinet meeting.
If he does not appear, questions will be asked as to whether Mr Yar'Adua is fit enough to resume his duties.
The BBC's Ahmed Idris says two planes arrived amid tight security at Nnamdi Azikiwe International Airport in Abuja at about 0200 local time (0100 GMT).
Although there has been no official confirmation of Mr Yar'Adua's return, two cabinet ministers told our correspondent the president was on board one of the jets.
And Nigerian Senator Abubakar Umar Gada told the BBC's World Today: "I can confirm from credible sources the president is back. I think Nigerians will be happy to have their president back in a good shape."
Our correspondent says everyone except presidential officials was kept out of the airport.
Not seen in public
He followed the official convoy the 40km (25 miles) into central Abuja and says there was a military presence every 300m on the route, which became more stringent towards the city centre.
Mr Yar'Adua's state of health is unclear, and it is not known whether he wants to return to his post.
He was flown to Jeddah after falling ill last November to be treated for pericarditis, an inflammation of the membrane protecting the heart.
The president has not been seen in public since then and there has been no official report on his health condition.
A delegation of Nigerian ministers had travelled to Saudi Arabia on Monday for an update on Mr Yar'Adua's health.
The BBC's Caroline Duffield in Abuja says in theory he is able to take up the reins of power once more by writing a letter to parliament and telling them he has returned, although it is understood that no such letter has been received.
She says that in his absence, Nigeria's political elite has been consumed by a power struggle between Yar'Adua loyalists, who wished to keep him in power, and those who argued he was too ill to govern.
Ever since the installation of the vice-president, there have been reports of splits in the cabinet and stories that the president's allies were determined to bring him home, our correspondent adds.
Politically, she says everything depends on whether Mr Yar'Adua is fit enough to govern Africa's most populous nation, with 150 million people.
Sonala Olumhense, a columnist with Nigeria's Guardian newspaper, told the BBC's Network Africa programme Mr Yar'Adua's apparent return was a big surprise.
"In the past week or so we have been told that the acting president will now take the country in a new direction, give this country new energy.
"And suddenly the president himself turns up. If he's anywhere near capacity, and I would think capacity for him would be about 50-60%, I think he would want get back to work right away.
"There seems to be on his part or on the part of the people around him the consciousness that power is slipping away from them and they want it back."
In Mr Yar'Adua's absence, acting President Jonathan has been reshuffling ministers, pledging to tackle power shortages and forging ahead with an amnesty for militants in the oil-rich Niger Delta.
Some politicians have said he could win support to run for president in elections due by April next year.
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