In the deadliest attack, late on Monday, gunmen opened fire at a market in the town of Damboa, targeting local hunters who sell bush meat from animals such as monkeys and pigs, which strict Muslims are forbidden to eat, a local official said.

"Gunmen suspected to be members of BH (Islamist sect Boko Haram) came to the town market and shot dead 13 local hunters on the spot while five others died from their injuries at the hospital," Alhaji Abba Ahmed said.

"They came to the market in a Volkswagen Golf car, carried out the operation and left."

In a separate attack in the north's biggest city of Kano, some 500 km (310 miles) west of Damboa, on Tuesday, suspected Boko Haram members riding on motorbikes shot dead five people playing an outdoor board game, witnesses and a hospital source who received the bodies said. Two others were wounded.

Damboa is in the remote northeast, the sect's heartland near the borders with Niger, Cameroon and Chad.

Boko Haram wants to carve an Islamic state out of Nigeria, a country of 170 million people split roughly evenly between Christians and Muslims. The insurgency is seen as the top security threat to Africa's leading oil and gas producer.

Nigeria plans to deploy around 1,200 troops as part of a West African intervention force to combat Islamist militants occupying the north of Mali, and officials fear Nigeria's involvement could further inflame its own insurgency.

President Goodluck Jonathan told Reuters in Geneva on Tuesday that tackling global jihadists is in Nigeria's interest because of the links between its Islamists and those in the desert states to the north, like Mali. 

An Islamist group known as Ansaru, which has been blamed for abducting and killing Westerners, claimed responsibility for an attack on Nigerian troops heading to Mali on Sunday that killed two officers.

Boko Haram's long-bearded members practice a strict Wahhabist version of Islam that regards anyone who disagrees with it as infidels. They frequently target bars and other forms of entertainment.

Militants have killed several hundred people in the past three years in a campaign to impose sharia, Islamic law, on Nigeria. Their targets include the security forces and churches, although they have killed more Muslims than Christians.

Gunmen fired on the convoy of one of Nigeria's most senior Islamic leaders in Kano on Saturday, killing at least four people.