At least six people were killed on Wednesday in an explosion in the northeast Nigerian city of Maiduguri, a civilian vigilante assisting the military against Boko Haram and a witness told AFP.

"We have so far recovered six dead bodies. We are still working at the scene, so the casualties may rise," said Danlami Ajaokuta of the blast on the Baga Road, which happened at 5:30 pm (1630 GMT).

Nigerian President Muhammadu Buhari said during a visit to Niger's capital Wednesday that he was seeking help from neighboring countries to eradicate the Islamist group Boko Haram before they do "more serious damage."

"I'm coming to seek the support of our neighbors. I came first to Niger, tomorrow I will go to N'Djamena and I will also talk with the president of Cameroon, to seek out ways and means to reinforce our efforts," said the recently-elected president during his first official trip abroad. 

Amnesty International said Wednesday there was sufficient evidence for the International Criminal Court to probe senior Nigerian officers for war crimes in the battle against Boko Haram, prompting an angry rejection by the country's military. 

The group stated the case against five senior officers in a new 133-page report based on hundreds of interviews, including with military sources, and using leaked defense ministry documents. 

A soldier in fatigues furiously pedals his bicycle through the alleyways of Diffa, puffing away after the biggest town in southeast Niger banned motorcycles -- vehicle of choice of jihadists.

Situated on the border with northeastern Nigeria, the heartland of Boko Haram insurgents, Diffa is on high alert since it was attacked in February.

Thirteen people were killed on Tuesday in a suicide attack at a busy cattle market in the northeastern Nigerian city of Maiduguri, the Red Cross and civilian vigilantes battling Boko Haram said.

The blast in the Borno state capital happened at about 1:00 pm (1200 GMT) as traders were wrapping up business for the day, Shettima Bulama said in an account backed up by another local vigilante.

Muhammadu Buhari is to attend this weekend's G7 summit in Germany in his first major international meeting as Nigerian president, his spokesman told AFP on Tuesday.

"The President has accepted the invitation to attend the G7 summit. He's expected to depart Nigeria on Sunday for the summit. It's going to be a two-day trip," said Shehu Garba.

Boko Haram militants on Tuesday launched a renewed attack on Maiduguri in northeast Nigeria, just days after the military repelled an assault on the key city.

Residents said the Islamists arrived in the Moronti area of the city by river but were unable to advance further because of wide ditches and embankments dug by soldiers around the city limits.

Nigeria's new President Muhammadu Buhari will this week make his first foreign trip since taking office, traveling to Chad and Niger for talks on Boko Haram, his spokesman said on Monday. 

Shehu Garba said the two-day trip starting on Wednesday will focus on "matters of security", with the cooperation of Nigeria's neighbors seen as crucial to ending the Islamist uprising which has claimed more than 15,000 lives since 2009. 

Sixty-nine people burnt to death in southeast Nigeria after a petrol tanker lost control, rammed into a busy bus station and burst into flames, the Red Cross and police said Monday.

"The lorry lost control and ran into a motor park (bus station) near (a) roundabout in Onitsha," the commercial capital of Anambra state, area police spokesman Uchenna Ezeh said of Sunday's crash.

Nigeria's anti-corruption agency has arrested six senior officials from the country's central bank and 16 from commercial banks over alleged currency fraud, a spokesman told Agence France Presse on Monday.

"Six senior officials of the CBN (Central Bank of Nigeria) and 16 others from various commercial banks across the country were arrested over currency fraud," said Wilson Uwujaren.