Sifiso Dabengwa said he was stepping down in the interests of MTN, after the Nigerian government issued a $5.2 million (4.8 billion euros) fine on the South African telecoms giant.

"Due to the most unfortunate prevailing circumstances occurring at MTN Nigeria, I, in the interest of the company and its shareholders, have tendered my resignation with immediate effect," the CEO said.

The Nigerian Communications Commission (NCC) issued the fine after the company failed to deactivate 5.1 million unregistered SIM cards by the specified deadline. Nigeria is MTN's largest market, with about 60 million subscribers there.

Security and financial motives

MTN, one of Africa's largest mobile phone operators, has seen its share price fall by at least 17 percent since the fine was imposed. The NCC has said the sale of unregistered SIM cards poses a huge security risk to Nigeria, as many of those cards fall into the hands of militant groups like Boko Haram, which has stepped up its attacks in the country in recent months.

Some observers, however, say the government is acting more out of financial interest. "$5.2 billion would bring in a huge chunk of revenue" for the government, South Africa-Nigeria Chamber of Commerce head Dianna Games told the AFP news agency.

MTN has temporarily put Phuthuma Nhleko in charge as it seeks a permanent successor to Dabengwa.

blc/jil (AFP, dpa)