Three Vietnamese tourists and an Egyptian tour guide were killed when a blast hit their bus near the Giza pyramids in Egypt, authorities said.
At least nine other Vietnamese tourists were injured, as well as the Egyptian driver, according to official statements.
The country's Interior Ministry said the bomb was likely hidden near a wall on a street in Giza, in the outskirts of Cairo.
The incident took place at 17.15pm CET less than four km (2.5miles) from the world famous pyramids.
Dozens of police and firefighters were on the scene and pictures show one side of the bus badly damaged with the windows blown out.
Lan Le, 41, who was on the bus but uninjured said the tourists were on their way to a sound and light show.
"We were going to the sound and light show and then suddenly we heard a bomb. It was terrible, people screaming," she said. "I don't remember anything after."
The ministry confirmed two deaths and a state prosecutors office later said a third tourist had died.
Prime Minister Mostafa Madbouly told local media that the guide had died from his injuries.
"The bus deviated from the route secured by the security forces," Madbouly said.
"We have been in contact with the embassy of Vietnam to contain the impact of the incident, and what is important now is to take care of the injured."
This marks the first deadly attack against tourists in the country for over a year.
Egypt's tourism sector was recovering from a drop in visitors since an uprising in 2011.
Shortly after the explosion, a lorry was seen towing the bus away and the injured were taken to Al Haram hospital.
The last deadly attack on foreign tourists in Egypt was in July 2017, when two Germans were stabbed to death in the Red Sea resort of Hurghada.