Cairo, Dec 29 (IANS): Egyptian security forces killed 40 suspected extremists during operations in several parts of the country in response to an attack on a tourist bus a day before that claimed four lives, officials said on Saturday.

The operations were carried out in the governorate of Giza and the Sinai Peninsula after an explosion on Friday targeted a bus carrying Vietnamese tourists on an excursion to the Giza pyramids, leaving four people dead and 11 others wounded, Efe news reported.

Three of the tourists were killed along with the tour guide, an Egyptian citizen, according to Egypt's Attorney General Nabil Ahmed Sadeq. The Egyptian bus driver and 10 Vietnamese tourists were injured.

No terror group has claimed responsibility.

The country's Interior Ministry said that the explosion took place when a makeshift explosive device placed next to a wall on Al-Marioteya Road was detonated as the bus went past.

Vietnam's Foreign Minister and Deputy Prime Minister Pham Binh Minh said he was "deeply saddened" by the attack.

Vietnam's Foreign Ministry said on Twitter that it proposed the Egyptian government to grant emergency visas to the relatives of the victims.

Egyptian Foreign Ministry spokesman Ahmed Hafez called the attack an act of terrorism. "The bombing of the tourist bus in El-Maryoutiya, a despicable, cowardly terrorist act which targets what cannot be targeted: The determination of Egypt and the Egyptians," he tweeted.

This was Egypt's first attack against tourists using explosives since late 2015, when a bomb blew a Russian airliner out of the skies above the Sinai Peninsula, killing all 224 people aboard.

The country has been under a state of emergency since April 2017, following a series of terror attacks against Coptic Christian churches in the Nile delta.