Three South Koreans and an Egyptian driver were killed Sunday in the explosion on a tourist bus in the south Sinai, the Egyptian Interior Ministry said.

It said the bomb went off near the Taba border crossing with Israel. The bus had come from ancient St. Catherine's Monastery, a popular tourist destination in central Sinai, and was traveling to Israel, according to officials.

Security officials said the source of the explosion was not clear, but was likely to have been a car bomb or a roadside bomb detonated by remote control.

They said almost all 33 passengers on the bus were wounded by the explosion and were being treated in hospitals either in Egypt or across the border in the Israeli port city of Eilat.

No one immediately claimed responsibility for the attack, but Islamist militants have been waging a deadly insurgency against the military and police in the largely desolate region since the overthrow of Islamist President Mohammed Morsi in July.

If the militants were behind Sunday's attack, it would mark a shift in strategy, as tourists have not been targeted so far.

Tourism across Egypt, a vital earner for the country, has been severely hit by nationwide unrest since Morsi's ouster.

tj/hc (AFP, AP, Reuters)