Muslim extremists in Kenya brutally murdered four Christians - one of them mentally challenged - after they refused to recite the Islamic prayer of faith.

Persecution watchdog International Christian Concern reported that on August 18, soldiers with the Islamic extremist group Al-Shabaab dragged Changawa Muthemba out of his home in Kasala Kairu, Lamu County and took him to the nearby home of his brother-in-law, Joseph Kasena, 42, where a 17-year-old neighbor, Kadenge Katana, also happened to be at the time.

The three men were held at machete point and ordered to recite the Shahada - "there is no god but Allah, and Muhammad is the messenger of Allah."

When none of them did, the attackers tied them up and hacked them to death. The Muslim extremists then went to the home of Joseph's older brother, Charo, who was mentally challenged, burning his body.

Joseph's wife, Caroline, reportedly watched everything happen and is "severely traumatized", according to a local source.

"Joseph was an elder at a local church. He did communal farming, worked as a cook in a local hotel and also served as a night guard at the home of an expatriate family, who had to leave the area due to the deteriorating security situation. His wife runs a small kiosk," said the source.

The source said the family of the 17-year-old boy, who was part of a local Catholic church, are "devastated" at his murder.

Kenya is ranked #18 on Open Door USA's World Watch List of 50 countries where it is most difficult to be a Christian. The outlet notes that Al-Shabaab, an affiliate of Al-Qaeda, has targeted Kenyan Christians for years, attacking churches, public places and buses.

Last month, the terrorist group beheaded nine Kenyans-some of whom belonged to a local church-in the Pandaguo area of Lamu West in an attack that targeted non-Muslim men only. Two days earlier, attackers killed four people (including police), also in the Pandaguo area, and in June beheaded nine in the coastal district of Lamu.

Last year,  al-Shabab gunmen attacked a bus full of teachers in the same region, pulling 28 non-Muslim passengers from the vehicle and shooting them point blank, according to the Guardian.

The following month, the BBC reported, the militant group did the same to non-Muslim workers at a quarry near the Somali border and violently killed both Muslims and non-Muslims during deadly attacks in Kenya.

Last April, the terrorist group stormed Garissa University College, gunning down two security guards before singling out Christian students and killing 148 of them.