Release International has warned that the situation in Jos, Nigeria, could "spiral out of control" unless the authorities step up security to protect Christians from violent attacks by Islamic extremists.

Its warning follows the murders of three journalists in the last week. Two of those killed worked for a Church of Christ in Nigeria publication, while the third reporter worked for Nigerian daily newspaper The Nation. Their deaths bring the total number of journalists murdered in Jos since 2008 to six.

"Release is also concerned about the killing of journalists. Release has investigated and reported well-planned attacks against Christian communities in the past and exposed them as pre-meditated acts of religious cleansing," said Release.

"Such attacks are continuing - there must be freedom to report the truth, so pressure can be brought to bear to prevent the violence."

Hundreds of people have been killed in sectarian violence in Jos this year. The region sits squarely between Nigeria's predominantly Muslim north and largely Christian south, making it a flashpoint for religious violence.

Release estimates that around 50,000 people have been killed in clashes since 2001, with an influx of jihadists from countries like Chad fuelling tensions.

It warned that attacks on Christians in the northern states governed by Islamic law may spread south.

"Jos is a potential tinderbox," it said. "Unless the government brings the violence under control, there could be an explosion of aggression that would spread throughout divided communities in the land."