According to officials, the explosion occurred at 6:15PM local time when the bus passed a section of wall used to hide the device from motorists and pedestrians.
Two of the killed tourists are from Vietnam.
This is a developing story.
"We have to know that it's possible that it would be repeated in the future".
The attack took place as Egypt's vital tourism industry has showed signs of recovery after years in the doldrums because of the political turmoil and violence that followed a 2011 uprising that toppled former leader Hosni Mubarak.
In July 2017, two German tourists were stabbed to death by a suspected jihadist assailant at the Egyptian Red Sea beach resort of Hurgada.
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Mount Etna has been particularly active since July, with experts in Italy saying the volcano has been growing faster recently. Debris sits on the street in front of a heavily damaged house in Fleri, Sicily Italy, Wednesday, Dec. 26, 2018.
In October 2015, a bomb claimed by a local affiliate of Daesh killed 224 people on board a passenger jet carrying Russian tourists over the Sinai peninsula.
Security is already tight in Egypt, with the tourist season at its height and the country's main Christian minority, the Copts, preparing to celebrate Orthodox Christmas on 7 January.
Tourism has slowly started picking up.
Ten of the wounded were Vietnamese tourists, and one was the Egyptian driver, according to official statements.
But that figure was still far short of the record influx in 2010 when over 14 million visitors flocked see the country's sites.
Egypt's army and police launched a major campaign against militant groups in February, targeting the Sinai Peninsula as well as southern areas and the border with Libya.