President Goodluck Jonathan is under growing pressure from top-level corruption in Nigeria's oil industry, with the central bank asking what happened to $50 billion in missing oil revenues and his political mentor, former president Gen. Olusegun Obasanjo complaining about massive fraud in the industry.

Meantime, massive oil theft in the Niger Delta, the southern region where Nigeria's oil production is centered, is rising amid reports of growing politicization of militants who have been blamed for much of the theft in recent years.

They have threatened to bring oil production to a halt by 2015 unless the government and foreign oil companies compensate impoverished villagers for massive environmental damage in the region and introduce more equitable sharing of oil revenue.

The multifaceted oil issue is becoming a major problem for Jonathan, who hails from the Christian south himself, and is currently grappling with a mushrooming Islamist insurrection in the Muslim north.

He has been under growing public pressure for failing to take effective action against official corruption.

The West African state, whose 150 million people are roughly evenly split between Muslims and Christians, was once Africa's energy powerhouse.

But now it's losing an average of $5 billion a year in potential revenue because of sabotage and international criminal networks who are stealing around 150,000 barrels of crude oil a day.

The Royal Institute of International Affairs, a London think tank, reported in September this has reduced oil production to a four-year low of slightly less than 2 million barrels per day.

The massive theft costs the continent's second largest economy after South Africa as much as $1 billion a month.

Oil revenues provide around 80 percent of the state budget.

"Rampant corruption means little of this revenue actually makes its way back to the Niger Delta communities that host the industry, encouraging extortion and oil theft as alternative revenue streams," observed the Jamestown Foundation, a Washington think tank.

This is also incensing militants of the Movement for the Emancipation of the Niger Delta who have been waging an insurgency since 2005 and are now resurgent after a 2009 government amnesty halted their operations for a time in the labyrinthine creeks and swamps of the delta where foreign oil companies operate.

The massive and systematic oil theft, bolstered by deep-rooted official corruption in the West African producer involving well-connected officials and security personnel is fueling instability in the south amid the carnage taking place in the north.

The complaints by the Central Bank of Nigeria and Obasanjo, who's also leader of the ruling People's Democratic Party, have battered Jonathan at a critical time, as he seeks re-election in early 2015.

Previous polls were preceded by sharp increases in spending and official largess as politicians sought to buy their way to power.

In October, Nigeria launched a campaign to pressure Lichtenstein to return $254.7 million stashed by former military dictator Gen. Sani Abacha, the most brutal of Nigeria's military rulers.

The funds are still stashed in the tiny European principality 14 years after recovery proceedings began following Abacha's death in 1998.

A leaked text of the central bank's letter to Jonathan says the state oil company has failed to account for nearly $50 billion in crude oil sold from January 2012 to July 2013 that should have been remitted to government coffers.

The Financial Times reported the shortfall added up to 76 percent of crude sold by the Nigerian National Petroleum Corp. and is nearly equal to federal budget expenditure for both years. The NNPC rejects the allegations.

Obasanjo said he felt compelled to confront Jonathan with his complaints -- which may have more to do with pre-election positioning than genuine outrage at the massive rip-off of state funds -- out of "serious concern" about the direction in which Africa's most populous nation is heading under the man he helped propel to high office.

He urged his former protege to stand down "before it is too late" -- read: save the party embarrassment at the polls -- in the interests of national unity.

In his 18-page letter, Obasanjo trotted out a litany of alleged fiscal abuses in government that included the sale of 430,000 barrels of oil he says were not remitted to federal coffers.