Three Vietnamese holidaymakers and an Egyptian guide were killed Friday when a roadside bomb blast hit their tour bus as it traveled close to the world-famous Giza pyramids outside Cairo, officials said. A statement by the public prosecutors office said 11 other tourists from Vietnam and an Egyptian bus driver were wounded when the homemade device exploded.   The improvised explosive device was placed near a wall along the Mariyutiya Street in Al-Haram district near the Giza Pyramids, it said.  The bus was carrying a total of 16 people including 14 Vietnamese tourists, an Egyptian driver and a tour guide, according to the statement.  Armed security personnel quickly deployed to the site and cordoned off the area for inspection.  The white tourist bus could be seen with its windows shattered and surrounded by soot-covered debris. Shortly afterward, workers brought a pick-up truck to tow the bus away. Prime Minister Mostafa Madbouli visited the injured tourists at Al Haram hospital, where he announced that the tour guide had succumbed to his injuries. Madbouli urged against "amplifying" the incident as he insisted that "no country in the world can guarantee that its 100 percent safe". "Its possible at times that an individual incident takes place here or there," he told journalists. "We have to know that its possible that it would be repeated in the future."  There was no immediate claim of responsibility for the blast.  The tourists were heading to a sound and light show at the pyramids, which they had visited earlier in the day, said Lan Le, 41, who was also aboard the bus but unhurt. "We were going to the sound and light show and then suddenly we heard a bomb. It was terrible, people screaming," she told Reuters, speaking at Al Haram hospita. "I dont remember anything after." Vietnams foreign ministry confirmed on Saturday that three of its citizens had been killed in the blast, and that it was working closely with Egypts interior ministry to provide assistance to the injured and to their families. The attack will likely prompt authorities to further tighten security around churches and associated facilities ahead of the New Years Eve celebrations and next months Christmas celebrations of the Coptic Orthodox Church.