Nigeria's military said Friday it had killed 50 Boko Haram Islamists in an operation in the northeast launched in response to an insurgent attack, in the latest violence to hit the region.

Gunmen suspected to be from the violent Islamist sect opened fire in a market on Thursday in the town of Gajiran, killing 15 people, residents said.

The military sent troops in pursuit of the attackers backed by air support, the area military spokesman Sagir Musa told journalists in the northeastern city of Maiduguri.

"Troops pursued the terrorists to their camps with air support and about 50 insurgents were killed in a shootout," Musa said.

"Troops are pursuing the remnants of the fleeing terrorists by blocking all possible escape routes," he added.

The violence occurred in Borno state, Boko Haram's historic stronghold and where the military has battled insurgents for the past four years.

Most of the northeast was placed under a state of emergency in May, when the military launched a major offensive aimed at crushing the Islamists.

The phone network has been shut down in Borno since the emergency measures were imposed and details of both Islamist attacks and military operations have been slow to emerge and difficult to verify.

In Gajiran, roughly 50 kilometres (30 miles) from Maiduguri, Borno's capital, dozens of Boko Haram gunmen reportedly pretended to be traders looking to sell goods in the local market.

They then spread into groups, spraying civilians with gunfire, while separately setting fire to a local government building and a police post.

The military has claimed major gains in the offensive, describing the Islamists as being in disarray and only capable of attacking soft targets in remote areas.

While the number of attacks appeared to decline shortly after the military campaign was launched, the bloodshed has spiked in recent weeks, with dozens of people killed in a range of attacks across the region.

Muslim worshippers have been slaughtered while gathering for morning prayers and more than 40 students were killed in an attack on a school in July, among other incidents.

Some of Boko Haram's recent attacks have targeted vigilante groups that have formed in Borno to defend the population.

The vigilantes have set up check points and carried out citizens' arrests on individuals they claim are insurgents.

Visiting Maiduguri on Friday, Nigeria's chief of army staff, Azubuike Ihejirika, praised the vigilantes, calling them "youth volunteers" who have "greatly supported the Nigerian army."

Some civilians have also voiced support for the militia groups, but many warn that they could make the crisis worse.

Such groups have been previously used in Nigeria as political enforcers.

Boko Haram claims it is fighting to create an Islamic state in mostly Muslim northern Nigeria.

The southern half of the country, Africa's most populous, is mainly Christian.

Despite Nigeria being Africa's top oil producer, most Nigerians live on less than $2 a day and poverty is most acute in the north.

Analysts say improving living conditions in the north is the only way to permanently end the insurgency.

The Boko Haram conflict is estimated to have cost more than 3,600 lives since 2009, including killings by the security forces, who have been accused of major abuses.