Nigeria has ordered Africa's leading mobile operator MTN to pay back taxes of $2bn just days after the government ordered it to repatriate $8.13bn, the company said on Tuesday.MTN gave a brief statement to the market in which it confirmed that Nigeria's attorney general gave "notice of an intention to recover the $2bn from MTN Nigeria"."Based on the detailed review performed, MTN Nigeria believes it has fully settled all amounts owing under the taxes in question," said the company.Nigeria's central bank said on Wednesday that it had ordered South Africa-based MTN to refund $8.13bn that it allegedly illegally repatriated and fined four banks involved in the transfer. News of the unexpected tax bill sent MTN shares tumbling by as much as 7.5% to an almost 10-year low. It was also the latest challenge to hit MTN since it began operations in Nigeria 17 years ago.MTN was fined $5.2bn in 2015 by Nigeria's telecoms regulator for failing to disconnect unregistered SIM cards on its network. The fine was later reduced to $1.7bn after a series of negotiations with the Nigerian government.* Sign up to News24's top Africa news in your inbox: SUBSCRIBE TO THE HELLO AFRICA NEWSLETTERFOLLOW News24 Africa on Twitter and Facebook.