South African mobile phone operator MTN will have to pay a $3.9 billion fine imposed by Nigeria for failing to disconnect users with unregistered SIM cards by Dec. 31, a source in the Nigerian telecommunications regulator said on Wednesday.

Nigeria's telecoms regulator had cut the fine from an initial $5.2 billion after weeks of lobbying by Africa's biggest mobile phone company to get it reduced.

"Appropriate action will be taken," should MTN fail to meet the deadline, the source said, asking not to be named and giving no further details.

MTN said this month it would challenge the decision in court.

Nigeria has been pushing telecoms firms to verify the identity of subscribers amid worries unregistered SIM cards were being used for criminal activity in a country facing the insurgency of militant Islamist group Boko Haram.

The fine came months after Muhammadu Buhari swept to power in Africa's biggest economy following a campaign in which he promised tougher regulation and a fight against corruption.