President Umaru Yar'Adua was out of Nigeria for three months
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 Abuja airport in the early hours, 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 not attended the weekly cabinet meeting, and it is unclear if he is well enough to resume duties.
Caroline Duffield, BBC News, Abuja
For President Yar'Adua's political future, it is critical the people do see him, and soon.
He and his circle have suffered political damage and a massive loss of trust, within the country, within parliament, and within their own party.
In his absence, many senior cabinet ministers - including the information minister and the foreign minister - were locked out of decision-making.
It created an atmosphere in which people were uncertain whether the president was even alive.
Mr Yar'Adua's allies will want executive powers transferred back to him, but there are cabinet splits.
Acting President Goodluck Jonathan's supporters have been manoeuvring to try to formalise his position.
The prize - control of Nigeria's oil wealth - is immense.
The 58-year-old leader has been receiving treatment for heart and kidney problems.
An hour after Wednesday's cabinet meeting was due to begin, ministers were reportedly still waiting inside the presidency, uncertain who was to chair the session.
Acting President Jonathan then postponed it, announcing to ministers that he would hold a special briefing at 1400 (1300 GMT), Reuters news agency reported.
Analysts say there has been considerable jostling for power behind the scenes as politicians look ahead to elections due by April next year.
The BBC's Ahmed Idris says two planes arrived amid tight security at the presidential wing of 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."
Presidential adviser Emmanuel Egbogah told Reuters Mr Yar'Adua was back, but it was "doubtful" he would resume office immediately.
Not seen in public
Nigeria's ambassador to Saudi Arabia, Abdullah Aminchi, told AFP news agency Mr Yar'Adua had returned home and his condition had "improved tremendously".
Our correspondent says everyone except presidential officials was kept out of the airport.
He followed the official convoy the 40km (25 miles) into central Abuja and says there was a military presence along the whole route.
The president was flown to Jeddah after falling ill last November to be treated for pericarditis, an inflammation of the membrane protecting the heart.
He has not been seen in public since then and there has been no official report on his health condition.
On Monday, a delegation of Nigerian ministers had travelled to Saudi Arabia, apparently 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 the National Assembly and telling them he has returned, although it is understood that no such letter has been received.
Acting President Goodluck Jonathan has adopted the mantle of power
Sonala Olumhense, a columnist with Nigeria's Guardian newspaper, told the BBC's Network Africa programme he believed Mr Yar'Adua would be anxious to return to work, even if only semi-recovered.
"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," he said.
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.
The amnesty was seen as Mr Yar'Adua's major achievement since coming to power in 2007, but it had been jeopardised by his absence.