Nigerian authorities have seen documents suggesting the proceeds from past crude oil sales were diverted to personal accounts rather than reaching government coffers, President Muhammadu Buhari said in a wide-ranging interview.

Africa’s biggest economy faces its worst economic crisis in years, since it relies on oil exports for about 58 per cent of government revenue. The sharp fall in oil prices over the past year has hit those revenues hard.

This problem has been exacerbated by the long-standing mismanagement of oil revenue. Buhari has previously said treasury coffers were virtually empty when he took office in May and that “mind-boggling” sums of money had been stolen.

The 73-year-old former military ruler, who won April elections after campaigning on an anti-corruption ticket, outlined progress made by his government in a two-hour “media chat” with three journalists broadcast live on state television.

“We have some documents where Nigerian crude oil was lifted illegally and the proceeds were put into some personal accounts instead of the federal government accounts,” said Buhari.

The president said stolen money had already been recovered by the government. He did not disclose the sums involved and said he could not provide more details because various cases were being taken to court.

A former oil minister, Diezani Alison-Madueke, is under investigation as part of a crackdown on corruption in the oil industry. She has denied any wrongdoing. Buhari’s latest comments suggested other officials might also be named.