Three Vietnamese tourists and an Egyptian guide have been killed and 11 others injured when a bomb blast hit their bus less than four km from Egypt’s world famous Giza pyramids, authorities said.

The blast is the first deadly attack against foreign tourists in Egypt for more than a year and comes as the tourism sector, a vital source of foreign currency revenue, was recovering from a sharp drop in visitor numbers since the country’s 2011 uprising.

No immediate claim of responsibility was reported. Islamist extremists including some linked to Islamic State are active in Egypt and have targeted foreign visitors in the past.

Nine of the injured were Vietnamese tourists, and one was the Egyptian driver, according to official statements.

The white tourist bus could be seen with its windows shattered and surrounded by soot-covered debris.

The interior ministry said the bus was hit by an explosion from an improvised device hidden near a wall on Marioutiya street early in the evening.

About two hours later the vehicle could be seen behind a police cordon with one of its sides badly damaged and the windows blown out, a Reuters reporter said.

Dozens of police, military and firefighters were at the site, on a narrow sidestreet close to the ring road, where traffic was moving normally.

Shortly afterwards, workers brought a pick-up truck to tow the bus away.

An investigator at the scene said the device had likely been planted near the wall.

The injured were taken to the nearby Al Haram hospital, where Prime Minister Mostafa Madbouly told local TV that the guide had died from his injuries.

“The bus deviated from the route secured by the security forces,” Madbouly told Extra News TV.

“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,” he said.

The interior ministry confirmed the death of two of the tourists, and the state prosecutors office later said a third had died. In total, 14 Vietnamese tourists had been traveling on the bus, it said.

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.

The government says fighting Islamist militants is a priority as it works to restore stability after the years of turmoil that followed the “Arab Spring” protests of 2011.

Those events and the bombing of a Russian airliner shortly after it took off from Sharm el Sheikh in 2015 caused tourist numbers to plunge.

Egypt’s tourism industry has been struggling to recover from terror attacks and domestic instability that has hit the country in recent years.

In July 2017, two German tourists were stabbed to death by a suspected jihadist assailant at the Egyptian Red Sea beach resort of Hurgada.

In October 2015, a bomb claimed by a local affiliate of the Islamic State group killed 224 people on board a passenger jet carrying Russian tourists over the Sinai Peninsula.

The incident dealt a severe blow to Egypt’s tourism industry still reeling from the turmoil set off by the 2011 uprising that forced veteran leader Hosni Mubarak from power.

Egypt has for years been battling an Islamist insurgency in North Sinai, which surged following the 2013 military ouster of president Mohamed Morsi.

Security forces have since February been conducting a major operation focused on the Sinai Peninsula, aimed at wiping out a local IS branch.

More than 450 suspected jihadists and around 30 Egyptian soldiers have been killed since the offensive began, the army said in October.

The Pyramids of Giza are the only surviving structure of the seven wonders of the ancient world and a major tourist draw attracting visitors from across the globe.