A Saudi teacher and an American principal of Palestinian origin were shot dead at a well-known private school in north Riyadh.
Security sources said that the armed attack left an Asian employee injured, according to Akhbaar24.
The police launched a manhunt to track down the killer, reportedly an Iraqi national who was dismissed from the school.
However, Al Ekhbariya TV channel said that the killer has yet to be identified.
"There has been a shooting at Kingdom School in Riyadh," the US mission said on its Twitter account, warning that the "shooter (was) still at school", according to AFP.
"School (is) closed, no children present. Please avoid area," the Twitter post said, without elaborating on casualties or motives behind the shooting.
There has been a shooting at Kingdom School in Riyadh, shooter still at school. School closed, no children present. Please avoid area.
— ACS Saudi Arabia (@KSA_ACS) May 31, 2017
Ongoing security incident at Kingdom School as of 3:00pm local time. Please avoid area due to heavy police presence. pic.twitter.com/Tc5Yq8tVY4
— ACS Saudi Arabia (@KSA_ACS) May 31, 2017
Kingdom School is currently closed for the summer holidays which started ahead of Ramadan.
The school is part of GEMS Global Network of schools. It is a school owned and operated by Kingdom Holdings, a company of Prince Al Waleed bin Talaal.