CAIRO - An explosion on a tourist bus in Egypt's Sinai peninsula killed three Koreans and the Egyptian driver on Sunday, the Egyptian Interior Ministry said, an apparent turning point in an Islamist insurgency that has gained pace since an army takeover in July. Several other Koreans were injured in the blast.The ministry said the bus was traveling from St. Catherine's Monastery, a popular tourist destination in the south Sinai, to nearby Israel when it was attacked. It did not state the cause of the blast. But two security sources said an explosive device planted either inside or near the bus was used.Magen David Adom was waiting to receive as many as 15 wounded tourists from the Egyptian side of the border and evacuate them to Eilat.Al-Qaida-inspired Islamist militants based in the largely lawless Sinai have stepped up attacks on security forces since army chief Abdel Fattah al-Sisi ousted elected president Mohamed Morsi of the Muslim Brotherhood in July.If militants were behind Sunday's attack, that would mark a shift in strategy to targeting tourists and economic targets and not just Egyptian police and soldiers. State television showed a photograph of the bus. Its windows were blown out and the roof had been partially torn off. Plumes of black smoke billowed from the site of the explosion on a palm tree-lined boulevard.Egypt has been rocked by political turmoil and violence since the downfall of Morsi, Egypt's first democratically-elected president, in July, after mass protests against him.Since he was deposed, Islamist militants based in the largely desolate Sinai near Israel have stepped up attacks on security forces, killing hundreds.Morsi appeared in court on Sunday on charges of conspiring with foreign groups to commit terrorist acts in Egypt.