South African telecoms giant MTN said Thursday it would launch a court challenge against the $3.9 billion fine that Nigeria has ordered it to pay by December 31 for failing to disconnect unregistered users.

Nigerian authorities ordered unregistered SIM cards to be deactivated for security reasons, as the country battles Boko Haram Islamists as well as criminality, especially kidnapping for ransom.

MTN was slapped with the hefty penalty by the Nigerian Communication Commission (NCC) in October after it missed the deadline to disconnect 5.1 million SIM cards.

“MTN Nigeria acting on legal advice has resolved that the manner of the imposition of the fine... is not in accordance with the NCC’s powers,” the company said in statement.

“There are valid grounds upon which to challenge the fine.”

It added that MTN lawyers would take the case to the High Court in Lagos.

The fine was subject to confusion after the NCC cut the initial $5.2 billion fine to $3.4 billion.

But it was then increased to $3.9 billion after the NCC admitted making a calculating error.

Johannesburg-based MTN – Africa’s largest telecoms firm – was fined 200,000 naira ($1,000) for each unregistered users.

It said it now expected all parties “to restrain from taking further action until the matter is finally determined.”

“The company will continue to engage with the Nigerian authorities to try and ensure an amicable resolution,” it added.

MTN shares gained 6.3 percent to close at 138.20 rand in Johannesburg.