KANO, Nigeria - Boko Haram have killed five traders in an ambush on a highway in northeast Nigeria that was recently reopened after coming under repeated attack from the Islamist group, a military source said on Wednesday.

A convoy of vehicles carrying Nigerian immigration officials and a group of traders were travelling from Gamboru, on the border with Cameroon, to the city of Maiduguri when they came under fire on Monday.

"Three (immigration) officers were hit and injured in the fire exchange. The immigration men drove off towards Maiduguri, leaving the traders behind," an army officer said on condition of anonymity.

The five traders had joined the official convoy thinking it would be a safer way to reach the Borno state capital than travelling the notorious road alone.

The traders’ two trucks were looted and set alight, Gamboru resident Umar Ari said by phone, also confirming the toll of five dead.

The Gamboru to Maiduguri highway is a strategic 140-kilometre trade route in the region, and provides an important link with neighbouring Cameroon.

The highway was closed for safety reasons at the height of the Boko Haram unrest some two years ago, but was reopened by the Nigerian authorities following a string of successes against the militants.

"Since the Maiduguri-Gamboru highway was reopened two months ago convoys of trucks carrying goods and other vehicles conveying traders have been coming to Gamboru under military escort without any incident," said resident Ari.

Boko Haram, which seeks to impose strict Islamic law in northern Nigeria, has been blamed for some 20,000 deaths and displacing more than 2.6 million people since 2009.

A counter-offensive spearheaded by Nigeria since January last year has recaptured swathes of territory lost to the Islamist militants in 2014.

But sporadic attacks continue and security remains a major concern in the region. — AFP