Nigerian authorities said Monday that a doctor in Lagos who treated a Liberian victim of Ebola has contracted the virus, the second confirmed case in sub-Saharan Africa's largest city.

"This new case is one of the doctors who attended to the Liberian Ebola patient who died," said Health Minister Onyebuchi Chukwu.

Cameroon's president has sent his army chief to the north of the country to beef up the battle against Nigerian Islamist group Boko Haram.

Following a series of cross-border incursions and kidnappings, President Paul Biya said Saturday he was also sending more troops and military supplies to the area.

A hijacked Singaporean oil tanker carrying 21 sailors was released on Sunday a week after being seized by pirates in waters off Ghana in west Africa, Seoul's foreign ministry said.

The 3,200-ton tanker, carrying a South Korean captain and chief engineer, was hijacked on July 26 off the southern coast of Ghana, the ministry said.

Cameroon's president has dismissed two senior army officers deployed in the country's restive north after at least 15 people died in attacks blamed on Nigeria's Islamist Boko Haram movement, state radio reported Wednesday.

A female suicide bomber killed six people at a college campus in Nigeria's Kano city on Wednesday, the fourth time Boko Haram Islamists were suspected of using a female attacker in as many days.

The latest violence came as the government announced the arrest of a 10-year-old girl with explosives strapped to her chest in a neighboring area.

Suicide bombers attacked two mosques in northeast Nigeria's Yobe state late Tuesday killing at least six people and injuring several others, witnesses told Agence France Presse blaming the Boko Haram Islamists.

Two blasts by female suicide bombers killed three people and injured 13 in Nigeria's Kano city on Monday, bringing the number of attacks this week in the area to five and overshadowing festivities marking the end of Ramadan.

The violence blamed on Boko Haram Islamists marred what was supposed to be a festive day in Kano, a city of more than six million people and the largest in Nigeria's mainly Muslim north.

At least five people were killed and eight were injured Sunday in a bomb attack on a Catholic church in a mainly Christian area of Kano, the largest city in Nigeria's north, police said, as five cops were hurt as they prevented a female suicide bomber from carrying out an attack outside a Kano university.

The church bombing came shortly after the end of mass.

Boko Haram Islamists have executed 12 people including a village chief in a remote part of northeastern Nigeria, residents said Saturday.

The attackers entered the village of Garubula, in Biu district, late Wednesday and dragged their victims out of their homes before shooting them, the residents said.

Nigeria was on alert against the possible spread of Ebola on Saturday, a day after the first confirmed death from the virus in Lagos, Africa's biggest city and the country's financial capital.

The health ministry said Friday that a 40-year-old Liberian man died at a private hospital in Lagos from the disease, which has now killed more than 650 people in four west African countries since January -- the deadliest outbreak in history.