Nigeria has ordered South African telecoms giant MTN to refund $8.13 billion that it allegedly illegally repatriated and fined four banks involved in the transfer.

The Central Bank of Nigeria said in a statement late Wednesday that MTN had not obtained approval before transferring the funds.

The regulator added that the banks had breached foreign exchange rules by failing to verify if the company had met all the requirements.

"The CBN has asked the managements of the banks and MTN Nigeria Communications Limited to immediately refund the sum of $8,134,312,397.63, illegally repatriated by the company to the coffers of the Central Bank of Nigeria," it said.

Nigeria said Standard Chartered, Citibank, Stanbic and Diamond banks were found to have helped MTN illegally repatriate the money between 2007 and 2015, in breach of Nigeria's foreign exchange and anti-money laundering laws.

MTN has denied the allegations.

“No dividends have been declared or paid by MTN Nigeria other than pursuant to certificates of capital importation issued by our bankers and with the approval of the CBN (Central Bank of Nigeria) as required by law,” MTN said in a statement.

The allegation is the latest setback for MTN in Nigeria, its most lucrative but increasingly most problematic market, coming two years after it agreed to pay $1 billion to end a long running dispute over unregistered SIM cards.

- Reporting by AFP and Reuters.