Muslim extremist group Boko Haram is continuing its reign of terror in northern Nigeria, with dozens more Christians killed as churches are relentlessly targeted.

Christian Solidarity Worldwide (CSW) reports that nine members of a local church in Attagara village in Borno state were killed on June 1 as gunmen opened fire on the congregation who had gathered for their usual Sunday morning service.

The church was then attacked two days later on June 3 to avenge the deaths of several Boko Haram militants who had been killed in the village, "practically razing" Attagara before attacking two other nearby communities.

Gwoshe Town, also in Borno state, was also subject to an attack on June 1, during which two churches were torched along with several homes and local shops. 21 Christians had reportedly already been killed in the town on 25 May when militants descended upon the Church of Christ in Nations.

Violence has been increasing in the region for over a year, with isolated communities reportedly suffering the most. Many churches have been burned to the ground, while others have been attacked with petrol bombs and other explosives. The Gwoza Chrisian Community Association wrote an open letter to the Borno State Governor in November 2013, accusing him of not acting quickly enough to stem the violence, and noting that 46 mainly Christian villages had been entirely destroyed by Boko Haram members.

More than 14,000 Christians have thus been forced to flee, some even across the border into Cameroon.

Chief Executive of CSW Mervyn Thomas has offered his condolences to the suffering communities, and has called on the Nigerian government to do more to protect its citizens.

"Gwoza LGA is clearly under a sustained assault and the unrelenting series of attacks provides further evidence, if any were needed, of Boko Haram's strategy of religious cleansing targeting of indigenous Christian communities of north-eastern Nigeria, as well as its appalling disrespect for traditional rulers," he says in a statement.

"This persistent targeting of communities on the basis of religion may amount to a war crime and is a clear violation of their right to freedom of religion or belief.

"It is vital that the Nigerian government ensures adequate protection so that that all of its citizens can exercise their right to freedom of religion or belief without the threat of abduction or death."

Boko Haram, meaning 'Western education is forbidden' has caused over two thousand deaths in Nigeria since 2009. It has connections with Al-Qaeda and has been designated a Foreign Terrorist Organisation by the US Administration. It is responsible for the kidnapping of over 200 young girls from a school in Chibok, also in Borno state, in April. The girls are yet to be rescued.