There is a "pressing threat" to Christians in Kenya and Tanzania as a result of "Islamic groups determined to establish their brutal version of Sharia law," a leading persecution charity said today.

"Islamist groups are gaining ground in Africa,' said Chief Executive of Release International, Paul Robinson.

"There is evidence to suggest they will become a growing force for instability in East Africa in 2015. The greatest risk to freedom of faith in the New Year comes from Islamic groups determined to establish their brutal version of Sharia law – whatever the cost to human life."

Robinson said that Iraq, where militant group Islamic State has been gaining ground, was one of the worst places in the world to be a Christian in 2014. "Faced with the stark choice of conversion or beheading, most left everything and fled," he said, adding: "Intolerant extremism poses the greatest threat to Christians in 2015."

Looking ahead to the next year, Kenya and Tanzania in particular have been identified as areas of increasing concern for Christians. Islamists in Somalia are extending their reach and "threaten to destabilise surrounding nations," a statement from Release said, noting that Christians there are "experiencing a new and brutal wave of violence."

Despite Christianity being the majority religion in Kenya, and Christians making up almost a third of the population in Tanzania, they are coming under increasing attack by extremists. Violence is also increasing in Nigeria, where Boko Haram has destroyed 185 churches in the northeast of the country, and killed an estimated 5,000 people since 2009.

It is thought that the violence in Nigeria will increase even further in the run up to the presidential elections in February 2015.

"Where these extremists take control, religious minorities face extreme brutality,' said Robinson. "This exodus and displacement of Christians from the Middle East and Africa will continue into 2015, wherever militants with their doctrines of intolerance and religious cleansing gain ground."

Another country highlighted for its hostility towards Christians was Sudan, where the government has been shutting down and destroying churches. North Korea, considered to the the most severe state persecutor of Christians in the world, was also mentioned, in addition to China, Vietnam and India.

A full overview of Release's forecast for 2015 can be found here.