Tumbling oil prices, Boko Haram insurgency, political tension threaten Nigeria's credit rating

Standard & Poor's Ratings Services warns that it might downgrade Nigeria's credit rating because of tumbling oil prices and political unrest.

The rating agency says there's at least a 50 percent chance it will reduce Nigeria's long-term credit rating — already a speculative BB-.

Nigeria relies on oil and natural gas for 70 percent of its tax revenue and 90 percent of its exports. S&P now expects oil prices to average $55 a barrel this year, down from a September forecast of $105 a barrel.

Lower prices will drag Nigeria's economic growth to 5 percent this year from 6.3 percent in 2014, turn a trade surplus into a deficit and increase government debt. Nigeria is also fighting Boko Haram insurgents and heading toward a contentious presidential election this spring.