A picture taken on February 16, 2014, shows the wreckage of a tourist bus at the site of a bomb explosion in the Egyptian south Sinai resort town of Taba. A bomb tore through a bus carrying South Korean tourists near an Egyptian border crossing with Israel, killing at least four people and wounding 13, officials said. (Str/AFP/Getty Images)

A powerful explosion ripped through a tourist bus in Egypt’s Sinai Peninsula on Sunday, killing four people in the deadliest attack on vacationers here in years. The attack raises concerns that extremist militants fighting the government may be expanding their targets to include civilians and foreigners.

The blast hit the bus Sunday afternoon while it was parked near the Israeli border in the resort town of Taba, sending black plumes of smoke into the sky and scattering debris across the highway, according to images broadcast by Egyptian state television.

Three South Koreans and the Egyptian driver of the bus were killed and 27 others were injured, the Health Ministry said. It was not immediately clear if the attack was from a roadside bomb or another device, according to security officials, and none of the radical groups operating in Egypt claimed responsibility for the attack.

But militants in both the Sinai and on the Egyptian mainland have used roadside bombs, explosive-laden cars and rocket-propelled grenades to target police and army vehicles in recent months, after the military’s ouster of Islamist-backed President Mohamed Morsi in July spurred a budding insurgency.

Prior to Sunday’s bombing, militants had directed their assaults exclusively toward military and police checkpoints and buildings, emphasizing in statements posted on Internet forums that their battle is with the “apostate regime” backed by the army. There had been no deliberate militant attacks on Egyptian civilians or foreigners.

The country witnessed a similar armed revolt from jihadists in the 1990s, when fighters regularly gunned down tourists in the capital, decimating the nation’s tourism industry.

In 2004, militants affiliated with Sinai’s native Bedouin tribesmen targeted the Hilton hotel in Taba and two other nearby beach resorts with a series of bomb attacks, killing 34 and injuring 159.

Egypt is already grappling with a sinking economy, and the number of foreign visitors has plummeted since an uprising deposed strongman Hosni Mubarak in 2011.

The current military-appointed government has pinned its legitimacy on a return to stability and economic revival. Sinai’s Red Sea coastline is a major draw for tourists, despite a rise in violence.

“The terrorist groups are taking a different approach in confronting the state,” said Kamal Habib, co-founder of the Egyptian Islamic Jihad, who along with the group renounced violence more than a decade ago. From attacks on security forces, “they are moving to tourism, a field that is one of Egypt’s primary sources of income. This is likely the beginning of a new phase.”

Israel closed the Taba border crossing Sunday to travelers wishing to enter Egypt but was allowing Israelis to return home, a border official said.

Aviv Oreg, a a security consultant and former head of the global jihad desk in Israel’s military intelligence, said Israel and Egypt have enhanced military cooperation in the past few months due to the rise in violence on the border.

Egypt-based jihadist groups have also launched attacks on Israel in the past. According to Egypt’s official Middle East News Agency, the explosion took place just 200 yards from the border crossing.

The Egyptian Interior Ministry said in an official statement on its Facebook page that the tourists on the bus had traveled from Cairo to the St. Catherine’s Monastery in South Sinai, and then later to Taba, where they were waiting to cross into Israel.

“The bus stopped a number of times” on the trip, the ministry statement said. But security officials did not say if a device had been planted along the way.

“It is clear the terrorists targeted the bus after gathering information and keeping track of its location and stops,” Habib said. “How could security forces allow the bus to travel without security?”

Ruth Eglash in Jerusalem and Sharaf al-Hourani in Cairo contributed to this report.