Senegal's President Macky Sall has promised to re-establish the truth after a BBC investigation revealed his brother had allegedly benefited from lucrative dealings in the gas sector.
The President's brother, Aliou Sall, has denied the claims, which he said were "totally false".
The president said he would not accept "false accusations".
The report said Aliou allegedly received a secret payment from a company which sold gas blocks to BP.
The investigation published by BBC Panorama and Africa Eye found that BP had agreed to pay Timis Corporation, run by the Romanian-Australian business tycoon Frank Timis, between $9bn (£7bn) and $12bn in royalty payments for its stake in the blocks.
It alleged that Timis Corporation made a secret payment of $250,000 to a company run by the President's brother.
Mr Timis has also denied any wrongdoing.
President Macky Sall was elected in 2012 on a pledge to fight corruption in Senegal.
A statement issued by the government said the "serious and false allegations" had "no foundation".
The President said people were trying to take advantage of newly discovered fossil fuel reserves.
"We know that where there is oil, some will try to destabilise the country," he said.
"I want the truth to be re-established and the government will do this now without delay," he said at Dakar's main mosque, after attending prayers for the Muslim holiday of Idd-ul-Fitr.
In response, the BBC said: "We reject this allegation and stand by our investigation. It was conducted to the highest editorial standards."
Opposition leader Ousmane Sonko has called on Senegalese people to "mobilise massively" until authorities take "appropriate measures".
The minister of justice has also spoken, saying it was a "private affair" that involved a private citizen, adding however that he didn't believe the accusations against Aliou.
One MP, Aymérou Gning, has told local media that a parliamentary inquiry could be opened.
Frank Timis "likes to call himself the godfather of West Africa," his former wealth manager, Philip Caldwell, told the BBC.
Mr Timis has business links in several African countries including Burkina Faso and Sierra Leone.