Four security officers have been killed and five others injured in a suicide attack outside the Prophet's Mosque in Saudi Arabia's Medina, Islam's second holiest city, the Saudi interior ministry said.

The bombing was the third attack to hit the kingdom on Monday, after blasts in cities of Jeddah and Qatif.

Photos posted on social media show smoke billowing from a fire outside the mosque where Prophet Muhammad is buried.

"Four security guards were martyred and five others wounded as a result of their opposition to the suicide attacker who detonated explosives near them as he was on his way to the mosque," the ministry said on Twitter.