An explosion that ripped through a bus carrying South Korean tourists in Egypt's Sinai Peninsula, killing four people, at the weekend, signals a potential escalation in the fight by Islamist militants against the Egyptian government.
The insurgency that sprouted last summer had previously confined itself to targeting Egyptian military and police forces.
But as the government continues its broad repression of Islamists in the wake of the military's removal of Islamist President Mohammed Mursi in July, the attacks could turn into a much bloodier, guerilla-style conflict, analysts say. "This is more of a challenge to the government and the state's authority than there ever was before," said Kamal Habib, a founding member of Islamic Jihad, a group that was at the forefront of a similar revolt in Egypt in the 1990s but that later renounced violence.
There was no immediate assertion of responsibility for the bus attack, which killed three South Korean tourists - two men and a woman - and the Egyptian bus driver, the foreign ministry in Seoul said. At least 14 South Koreans were wounded.
"We are shocked and enraged at the terrorist bombing on the bus ... and strongly condemn the act," the ministry said. The tourists were all members of the same church group from the central South Korean county of Jincheon.
Habib said the blast indicates that militants have adapted their strategy to try to cripple the government by hitting the country's vital and ailing tourism industry. The military-appointed cabinet has pinned its legitimacy on a return to stability and economic revival after three years of turmoil that began with the Arab spring uprising.
"This is likely the beginning of a new phase" of the conflict between militant Islamists and the state, Habib said.
The insurgency has steadily expanded since August, when Egyptian security forces launched a crackdown on Mursi's supporters, killing more than 1,000 civilians, according to human rights groups. Since then, the government has arrested thousands of people associated with Mursi's Muslim Brotherhood or other opposition groups and passed laws strictly limiting protests.
For their part, the militants have killed at least 100 police officers and soldiers since August.
The government has responded to the insurgency with increased raids in the northern part of Sinai, where many Islamist militants are based.
In addition, after a deadly car bombing at a security building in the Nile Delta in December, which was claimed by Ansar Bayt al-Maqdis, authorities declared the Brotherhood a "terrorist organisation". The two groups have no known connection, and the Brotherhood renounced violence decades ago.
"If you blame every Islamist in society, you will make an enemy out of all of them," said Safwat el-Zayat, a former brigadier general in the Egyptian army. "This is going to be a long war."
Additional reporting by Agence France-Presse
Major attacks targeting tourists in Egypt since early 1990s
February 26: Bomb in a central Cairo cafe kills a Turk, a Swede and an Egyptian. Foreigners also among 19 wounded.
October 26: Two Americans, a Frenchman and an Italian killed and two other tourists injured at Cairo's Semiramis hotel.
March 4: Jamaa Islamiyya claims an attack against a Nile cruise ship. A wounded German tourist later dies.
September 27: Two Germans and two Egyptians killed in the Red Sea resort of Hurghada.
October 23: Briton killed and five others wounded in two separate attacks on tourists in southern Egypt claimed by Jamaa Islamiyya.
April 18: 18 Greek tourists killed and 14 wounded in an attack outside a Giza hotel.
September 18: Nine Germans and their Egyptian driver killed when their bus is firebombed outside Cairo's Egyptian Museum.
November 17: 58 tourists among 62 people killed in an attack on Hatshepsut temple in Luxor claimed by Jamaa Islamiyya.
October 7: Israeli tourists among 34 people killed in three bombings in the Sinai resorts of Taba and Nuweiba. More than 100 wounded.
April 7: Blast in mediaeval Islamic heart of Cairo kills two French nationals and an American as well as the bomber.
April 30: One person dies and eight wounded by bomb near Egyptian Museum shortly before police kill a woman and wound another after they open fire on a tourist bus.
July 23: About 70 die in bombings in Red Sea resort of Sharm el-Sheikh.
April 24: At least 22 killed and 150 wounded in three blasts in the Red Sea resort of Dahab.
February 22: Bomb at Cairo's Khan el-Khalili bazaar kills a 17-year-old French girl and wounds more than 20, mostly tourists.
September 22: 19 foreign tourists and Egyptians kidnapped in remote corner of southwestern Egypt. They are freed unharmed 10 days later.
February 16: Three South Korean tourists and their Egyptian driver killed in Sinai bombing.