Nearly 50 people have been killed in Nigeria in an attack by militant Islamist group Boko Haram on a group of fish traders, a union leader says.

The head of the fish traders association said the incident happened in Nigeria's restive Borno State.

"Scores of Boko Haram fighters blocked a route linking Nigeria with Chad near the fishing village of Doron Baga on the shores of Lake Chad on Thursday and killed a group of 48 fish traders on their way to Chad to buy fish," Abubakar Gamandi said.

He said the insurgents set up a barricade at Dogon Fili, 15 kilometres from Doron Baga, and stopped a convoy of fish vendors about midday.

"The Boko Haram gunmen slit the throats of some of the men and tied the hands and legs of the others before throwing them into the lake to drown," Mr Gamandi said.

Doron Baga, 180 kilometres from Maiduguri, is the base of the Multi-National Joint Task Force (MNJTF), comprising troops from Nigeria, Chad and Niger fighting the Islamist group.

The MNJTF was formed in 1998 to fight trans-border crime but its mandate was expanded to counter the Boko Haram insurgency in the volatile region.

Mr Gamandi claimed the assailants killed all of their victims without using their guns.

"The attackers killed their victims silently without the use of the gun to avoid attracting attention from the multi-national troops," he said.

A military officer in Maiduguri confirmed the attack but said details were sketchy.

"We heard of the attack near Doron Baga but we don't have any details because the area falls under the operational jurisdiction of the MNJTF," the military officer said.

News of the attack was slow to emerge due to the destruction of mobile phone towers in the area by Boko Haram in previous attacks.

Boko Haram force fishermen out

Incessant Boko Haram attacks have disrupted fishing and farming along the shore of Lake Chad and forced many fishermen from Doron Baga to abandon fishing and turn to importing dried fish from neighbouring Chad.

Mr Gamandi said the Dogon Fili route provided the safest passage for traders from Doron Baga to Chad as other routes are infested with Boko Haram gunmen who rob and kill travellers.

What is Boko Haram? Boko Haram, which means "Western education is sinful", is loosely modelled on the Taliban movement in Afghanistan.

Boko Haram, which means "Western education is sinful", is loosely modelled on the Taliban movement in Afghanistan. The group considers all who do not follow its strict ideology as infidels, whether they are Christian or Muslim.

The group considers all who do not follow its strict ideology as infidels, whether they are Christian or Muslim. It demands the adoption of Sharia law in all of Nigeria. Read more

Last December at least seven fishermen were killed when Boko Haram Islamists attacked Doron Baga in a night time raid that left many homes burnt.

In August, the Islamists raided Dogon Baga and kidnapped 97 people after killing 28 villagers.

The hostages, including women and children, were loaded on speed boats and ferried across the lake into Chad.

Chadian troops rescued 85 of the hostages when they intercepted a convoy of buses transporting them from the shores.

Boko Haram was also responsible for the kidnap of 276 schoolgirls in the Nigerian town of Chibok more than six months ago.

Fifty-seven of the students managed to escape but talks aimed at securing the release of the rest of the group are at an impasse.

The Boko Haram violence has claimed thousands of lives since 2009 with the aim of creating a hardline Islamic state in Nigeria's mainly Muslim north.

Reuters/AFP