Speaking to Al Arabiya News Channel, Saudi Interior Ministry Spokesperson Mansour al-Turki confirmed the external guard post was located on a public street.
Saudi King Salman's Palace in Jeddah has been hit by a gunman, who shot dead two guards and wounded three others.
Al-Amri had in his possession three Molotov cocktails in addition to a Kalashnikov machinegun.
Amri disembarked from his vehicle at a checkpoint outside the western gate to the Al-Salam Palace and started shooting, according to the Saudi Press Agency.
The royal family usually conducts its official business in this palace during the summer months, according to Reuters.
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A gunman has attacked the Saudi royal palace in Jeddah, killing two guards and injuring three others.
The two guards killed were identified as Hammad bin Shallah Al Mutairi and Abdullah bin Faisal Al Subaie.
The US Embassy in Saudi Arabia has cautioned the American citizens in the kingdom, following reports of a foiled attack in the port city of Jeddah.
Investigations are still carried out by security authorities, and "any developments will be announced later", he added.
The intensified crackdown on ISIS militants followed the series of terrorist attacks across Saudi Arabia targeting security forces and Shi'ite Muslims.