Africa's biggest wireless operator was fined $3.9 billion in October after failing to disconnect 5.1 million subscribers amid concerns the lines were being used by Boko Haram insurgents.

In the letter dated February 24, MTN proposed paying 100 billion naira ($500 million) in five annual instalments, buying 80 billion naira ($400 million) of Nigerian sovereign debt, and offering 70 billion naira ($350 million) worth of access to its fibre network.

Last month, the Johannesburg-based company paid 50 billion naira ($250 million) to kick off settlement negotiations.

The offer is in the "shared interest in the development of the country's telecommunications sector and economy more generally," according to the letter to the Nigerian government from MTN's lawyer, former United States Attorney General Eric Holder.

Nigeria's Senate Committee on Communications met on Thursday to discuss the fine, saying that negotiations with MTN are expected to continue for two more weeks.

The Nigerian Communications Commission (NCC), the country's telecoms regulator, handed down the penalty last year citing security concerns over the inability to trace users in a country plagued by frequent kidnappings and Boko Haram.

"You know how the unregistered GSM (Global System for Mobile communication) are being used by terrorists," Nigerian President Muhammadu Buhari said earlier this week.

"That was why NCC (Nigerian Communications Commission) asked the MTN, Glo and the rest of them to register GSM," Buhari said.

"Unfortunately MTN was very slow and contributed to the casualties."

Boko Haram has killed over 17,000 people since embarking on a bloody quest to create an independent state in 2009.