Access to power and lucrative oil contacts are driving a rise in Christian-Muslim clashes and killings. Nigeria's heavy military presence is not enough to quell violence that plagues the No. 3 supplier of oil to the US.

Jos, Nigeria: Patience Dassah, a smartly dressed young Nigerian, has recently had trouble getting a taxi. But her trouble with okadas, the motorcycles that zip through the streets here in Africa's most populous country, does not lie with the typical traffic jams or fuel shortages.

"I live in a mostly Christian area, and now my Muslim okada driver will not take me there," she says, explaining that he is too afraid of being attacked or even killed. "He won't even pick up my calls.”

Ms. Dassah lives in Jos, a city in central Nigeria now caught in a cycle of religious and ethnic violence.

Sandwiched between the country's largely Muslim north and mostly Christian south, Jos has endured a decade of periodic clashes between followers of the two faiths, a conflict that has flared up in recent months.

In two massacres this year, gangs with machetes from both sides descended on nearby villages and killed hundreds.

A boosted Army presence and nighttime curfew in Plateau State, of which Jos is the capital, has reduced the risk of more large-scale attacks. But some residents of Jos are resorting to "secret killings," in which a lone Christian is lured to a secluded Muslim part of town – or vice versa – and killed.

They are the reason Dassah says her okada driver no longer answers her calls. And they appear to show once again that a heavy military presence is not enough to quell the religious violence that plagues the No. 3 supplier of oil to the United States.

Targeted killings of ethnic individuals: Secret killings have been on the rise since this year's second massacre, in early March, say Christian and Muslim community leaders, government officials, and police.

"We have started receiving reports of corpses being found here and there. It started happening after the March 7 incident," says Femi Oyeleye, the head of the state's criminal investigation department, referring to coordinated mob attacks by Muslims on members of the mainly Christian Berom ethnic group.

Those March killings are said to have been reprisals for religious clashes that killed scores in January.

According to Mr. Oyeleye, 25 corpses unrelated to any major attacks were found in Jos during March and April – far above the average rate of three corpses a month during the 12 months leading up to February.

Lawal Ishaq, a local lawyer who documents secret killings against Muslims, recorded 36 deaths in March and April.

Death tolls are sensitive in Jos – and highly unreliable.

The figures for the March massacre, in which mostly Christians were killed, range from 150 to 450 Similar uncertainty shrouds the January massacre, in which mostly Muslims died.

"Each side inflates the figures, and then the high numbers are used to justify revenge attacks," says Henry Mang, a researcher at Jos University's Centre for Conflict Management and Peace Studies.

Ethnicity and politics also play a role: In Jos, religion is just one cause of conflict. The fighting falls broadly along ethnic lines, with the mostly Christian Berom group against the largely Muslim Hausa and Fulani groups.

The Hausa and Fulani are officially deemed settlers in Plateau State, even though some have lived here for generations, and say that as a result they are excluded from political office.

The state government counters that the so-called settlers have been given a small number of roles and are now trying to take over.

Political office is prized throughout Nigeria – holders of such jobs can grant lucrative public contracts to their allies and access oil revenues in what is sub-Saharan Africa's biggest energy producer. "[The violence in Jos] is not about religion," says Murray Last, an anthropologist at University College in London and an expert on Nigeria. "That is just the glove that covers the hand. That hand is politics: the access to power and the access to land."

Faith-based ghettos emerging- As killings continue, and suspicion between Christians and Muslims grows, residents say the city is splitting into faith-based ghettos. Some say Berom farmers are afraid to travel to remote rural areas, missing the opportunity to plant crops during the rainy season.

In a sweltering car yard in the midday sun, Vincent Chungzi, a mechanic, fears that his missing cousin, a Christian okada driver, has become another victim.

He was last seen with a female Muslim passenger more than a month ago.

"We keep calling him, but he is not answering," says Mr. Chungzi. "His wife is crying all the time.”