Nigeria is seeking about $2 billion in back taxes from MTN Group Ltd., another curve ball directed at Africa’s biggest wireless carrier less than a week after it was ordered to refund $8.1 billion in illegally repatriated funds.

MTN outlined the tax dispute and refuted both accusations in a statement on Tuesday, yet faces an uphill battle to convince investors it won’t end up shelling out for either or both offenses in its largest market. The shares extended their slump, falling 17 percent to an almost 12-year low by the close in Johannesburg.

“This could be an economic and political play by Nigeria,” Ron Klipin, an analyst at Cratos Wealth in Johannesburg, said by phone. “The Nigerian economy is looking for additional sources of revenue and at the same time the government wants to be seen as tightening up the regulatory framework in the country.”

The additional scrutiny on MTN comes as Nigeria President Muhammadu Buhari seeks re-election for a new four-year term in a February vote. His administration has pledged to fight corruption in Africa’s most populous nation, including tax avoiders and companies acting unscrupulously. MTN is by far the biggest of Nigeria’s four wireless carriers, with about 54 million customers.

“We remain resolute that MTN Nigeria has not committed any offenses and will vigorously defend its position,” the Johannesburg-based company said.

The office of Nigeria’s attorney general calculated that MTN owes $2 billion related to the import of foreign equipment and payments to suppliers over the past decade. It asked the South African company to carry out a self-assessment in response, but last week rejected the company’s findings, which concluded that it had owed — and paid — $700 million. MTN reported the ongoing dispute for the first time Tuesday.

The difference between the $2 billion tax bill and the $700 million paid by MTN is due to different interpretations of charges ranging from VAT and withholding tax to import duties by the Nigerian government, said a person familiar with MTN’s thinking on the matter. The government assumed a flat rate on withholding tax, didn’t account for certain VAT exemptions, calculated an average duty on imports, whereas rates vary, and didn’t account for goods and services garnered from local suppliers, said the person.

Last week, the Nigerian central bank told MTN to return funds it alleges the company illegally transferred out of the country over eight years through 2015. That accusation put the carrier’s planned share sale in Lagos in jeopardy, while the sanctions may restrict its ability to pay dividends.

MTN Chief Executive Officer Rob Shuter said Monday the company is committed to Nigeria despite the turmoil. The former Vodafone Group Plc executive was appointed partly in response to a prior dispute in Nigeria over SIM-card registration, which resulted in an MTN payment of about $1 billion and a commitment to the Lagos IPO.

The shares closed at 72 rand, the lowest since November 2006, valuing the company at 136 billion rand ($8.9 billion).

“MTN is operating in frontier and emerging markets, and those markets are often risky and volatile,” Klipin said. BLOOMBERG