Nigeria landed MTN Group with a $2 billion tax bill on Tuesday, marking the South African mobile phone company's latest skirmish with authorities in its most lucrative market.

Last week Nigeria's central bank ordered MTN's Lagos-based unit to hand over $8.1 billion that it said was illegally sent abroad, raising questions about doing business in the west African country. Nigerian politics are seen by some analysts as a factor in the pressure on MTN. President Muhammadu Buhari, who swept to power in 2015 on promises to fight corruption and push through tougher regulation, is seeking re-election in next year's polls.

"The government of Nigeria is in dire need of cash from a budget perspective and MTN presents a good target because it's a big company and it has flouted regulations before," Dobek Pater of consultancy house Africa Analysis said.

The latest clashes with MTN come about two years after it agreed to pay more than $1 billion to settle a dispute over SIM cards in Nigeria, whose finances have been hit by a weak economy and volatile prices of crude oil.

But analysts the size of the demands against MTN risk further undermining Nigeria's efforts to shake off an image as a risky frontier market for investors.

MTN said in a statement detailing the case of the money sent out of the country that it had been in talks with Nigeria's Attorney General about an investigation into tax compliance.

"In this process, his office made a high-level calculation that MTN Nigeria should have paid approximately $2.0 billion in taxes relating to the importation of foreign equipment and payments to foreign suppliers over the last 10 years," it said.

MTN, whose Nigerian business brings in a third of its annual core profit, or EBITDA, said its total payment of around $700 million over the 10-year period fully settled the amount owing under the taxes in question.

Shares in MTN fell 17 percent to 72 rand at 1315 GMT, bringing losses since last Thursday, when the central bank issued its demand, to nearly a third. "These are old issues that have been investigated and closed but now they are being reopened," Byron Lotter, a portfolio manager at Vestact in Johannesburg, said, adding it was possible MTN would be reviewing its presence in Nigeria.

Vestact owns about 30 million rand worth of MTN stock.

PULLING OUT

South African hotels and casino group Sun International said it was in final stages of exiting Nigeria following clashes with regulators and shareholders. It is following in the footsteps of retailer Woolworths and foodmaker Tiger Brands.

MTN, which has expanded in more than 20 frontier markets including war-ravaged Syria and Afghanistan, called the latest demands by Nigerian authorities "regrettable and disconcerting". "We remain resolute that MTN Nigeria has not committed any offences and will vigorously defend its position," it said.

Nigeria's head of asset recovery in the attorney general's (AG) office could not immediately comment on the tax claim but said the AG's office, headed by Abubakar Malami, was also behind last week's $8.1 billion demand.

"We have a whistleblower who came forward with information that there are infractions and the attorney general triggered the investigation which was conducted by our regulatory central bank," Ladidi Mohammed told Reuters.

Nigeria's main allegation against MTN is that it used improperly issued certificates to convert shareholder loans in its Nigerian unit to preference shares in 2007. As a result, $8.1 billion in dividends paid by MTN Nigeria to its parent between 2007 and 2015 were illegal and should be returned.

- Reuters