A Boko Haram faction has killed four aid workers it kidnapped in July, a journalist with access to the terror group has said.

The aid workers are officials of the Action Against Hunger, Ahmed Salkida said in a tweet on Friday.

A fifth aid worker among the kidnapped, Grace Taku, who was the only female, has not been killed but condemned to "life of slavery," Mr Salkida said quotiing the Islamic State of West African Province (ISWAP), a breakaway faction of Boko Haram.

The journalist quotes the terror group as saying the execution was a result of a breakdown in talks with the government.

PREMIUM TIMES reported how the six aid workers were kidnapped on July 19 in Borno when armed gunmen attacked the vehicles used by AAH.

The victims later appeared in a video six days later, begging the Nigerian government to ensure their release.

One of them was killed in September by the terror group before today's murder of four others.

Neither the AAH nor the Nigerian government has reacted to the killing of the aid workers.

The AAH was recently reinstated into Nigeria after the Nigerian army controversially suspended its activities in the Northeast. The army accused the international aid organisation of assisting the terrorists, a charge the organisation denied.