Goodluck Jonathan was made acting president by MPs on 9 February
Nigeria's deputy leader Goodluck Jonathan is still the country's acting president while Umaru Yar'Adua is sick, the information minister has said.
Dora Akunyili was speaking amid confusion about who wields power in Nigeria after President Yar'Adua unexpectedly returned home.
The president's office quickly backed Mr Jonathan's position.
Earlier an official statement pointedly referred to Mr Jonathan as vice-president, inferring far less power.
Correspondents say a political struggle could be under way between rival power brokers.
In a clear statement, the information minister insisted that Mr Jonathan retained power.
Caroline Duffield, BBC News, Abuja
The surprise return of the president was a calculated fight-back staged by his closest allies.
They - and the vested interests behind them - have carefully watched Goodluck Jonathan take on the job of acting president. On returning, they publicly called him the vice-president.
Many people say this was an overt attempt to regain power. Others suggest it was a display - intended to warn Mr Jonathan that the ultimate power, the military, is with the Yar'Adua group.
Sceptics believe Mr Jonathan will now have to do a deal with the president's friends, and will serve as acting president only to do their bidding.
It is uncertain whether power really has shifted in this country.
"Dr Goodluck Jonathan, the acting president, is in charge," Ms Akunyili told Reuters news agency.
"There is no way he can go back to vice-president without going through the process that made him acting president."
The BBC's Caroline Duffield in the capital, Abuja, says the status of acting president is critical.
She says an acting president can sign legislation, chair cabinet meetings, reshuffle ministers and release oil funds.
Until he was formally made acting president earlier this month, Mr Jonathan had enjoyed only a ceremonial role since Mr Yar'Adua went for medical treatment in Saudi Arabia last November.
On his return on Tuesday, the presidency called him vice-president and only said he would "continue to oversee the affairs of state".
But in a special TV broadcast on Thursday, Mr Yar'Adua's spokesman Olusegun Adeniyi said Mr Jonathan remained Nigeria's acting leader and commander-in-chief.
"The acting president has already called us for a meeting tomorrow to brief us on how things will work, that is how he will run the presidency including everybody so it is just one presidency," he was quoted as saying.
Tussle for power
Mr Yar'Adua's condition is unknown and he has not been seen in public since 23 November.
No-one, not even Mr Jonathan, has seen the severely ill president since he returned to Nigeria, our correspondent says.
Behind the scenes, this is all about a ferocious scramble for power and access to Nigeria's vast oil revenues, she adds.
Nigerians fear Mr Yar'Adua is controlled by those around him. His wife, Turai Yar'Adua, has emerged as a dominating figure, and stands accused of running the government in all but name.
For two days no-one has been able to say who holds power and it remains unclear what should happen next.
"We do not think he [Mr Yar'Adua] has the capacity today," Osita Okechukwu of the Conference of Nigeria Political Parties, a group representing opposition parties, told the BBC on Wednesday.
"We are appealing to him, and his handlers, that he should honourably resign his appointment."
Late on Wednesday, the Senate sought to avoid a repeat of the constitutional uncertainty by voting to force future leaders who are sick to step aside after 14 days' absence from office.
However, this has to be approved by the House of Representatives and two-thirds of state legislatures.