The so-called Islamic State has released a video showing the execution of 20 men said to be Christians.

The video was released to the terrorist group's Amaq news channel.

The BBC Monitoring service said the executioners were from the Islamic State in West Africa Province (ISWAP) and that the captives came from Borno state in north-eastern Nigeria. 

IS called the executions a warning to "Christians around the world" and said that "jihadists will be at war with them ... till the end of times".

The video shows the men divided into three groups and shot dead by four masked militants.

After the second group is executed, one militant says, "We shall not rest until we take revenge for our brothers against the Christians all over the world."

According to the BBC, IS said that the killings had been carried out as "revenge" for the death of an IS spokesman and its former leader in Syria in February. 

Jo Newhouse, spokesperson for Open Doors in Sub-Saharan Africa, said the videos were intended to sow fear among Christians.

She praised the heroism of the men who were slain. 

"There are no words to describe the horror and heartbreak that the families of these men must be going through," she said.

"However, while ISWAP intends to sow fear with these videos, Christians around the world should be in awe of the testimony of these men who, to the best of our knowledge, held on to their faith even in the face of execution."

ISWAP is the same group that is holding Christian teenager Leah Sharibu captive. She was kidnapped from her school in Dapchi, Yobe state, five years ago and is continuing to be held after reportedly refusing to convert to Islam. 

Last month, ISWAP carried out a deadly attack on a market town in Iware, Taraba state, in the north-east of the country. In a message posted to Telegram, the group said the attack was on "a gathering of infidel Christians".

Newhouse said that the growing influence of violent jihadist groups in Nigeria was of "grave concern" and that "entire communities" were being "traumatized and uprooted".

"This video is a call for decisive action by the regional governments and the international community to address the multiple factors that create fertile ground for further radicalisation that allows these groups to flourish," she said.