Saudi Arabia's state-run news agency says security forces killed an 18-year-old Islamic State loyalist and arrested his brother after they killed their cousin, who is a soldier, and three other people.

The Saudi Press Agency said Saturday that security forces launched a manhunt after a video emerged purportedly showing an IS loyalist shooting and killing a man in the desert.

SPA says the individual who was shot dead was a member of the armed forces, lured to the desert by his cousins on Thursday, the first day of the Muslim holiday of Eid al-Adha.

It says the two brothers killed two other citizens and a corporal.

Security forces tracked the brothers to a mountainous area, where the brothers opened fire. A soldier was killed in the operation.