(Adds tourism minister's comment, number of wounded, background)
By Yasmine Saleh and Shadia Nasralla
CAIRO, Feb 16 (Reuters) - A bomb on a tourist bus in Egypt's Sinai peninsula killed at least two Koreans and the Egyptian driver on Sunday, army and security sources said, an apparent turning point in an Islamist insurgency that has gained pace since an army takeover in July.
The Interior Ministry said the bus was travelling 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, which wounded 20 people. Two security sources said a bomb planted either inside or near the bus was used.
"This is a terrorist act that was carried out with an explosive device," said an army source. "Terrorist" is the word used by security officials to describe Islamist militants.
Al Qaeda-inspired Islamist militants based in the largely lawless Sinai have stepped up attacks on security forces since army chief Field Marshal Abdel Fattah al-Sisi ousted elected President Mohamed Mursi of the Muslim Brotherhood in July after mass protests against his rule.
If militants were behind Sunday's violence, it would mark a shift in strategy to attacking tourists and economic targets and not just police and soldiers in Egypt, the Arab world's biggest nation. Egypt's tourism industry, a pillar of the economy, has already suffered from political turmoil and street protests.
Tourism Minister Hisham Zaazou said he was "very disappointed" by the attack, which revived memories of an Islamist insurgency in the 1990s.
"I hope this will be an isolated incident that will not reoccur, this is my wish. I'm reassuring that all the rest of the country is safe and secure and what happened can happen anywhere in the world," Zaazou told Reuters.
MURSI ON TRIAL
State television showed a photograph of the bus, its windows smashed and the roof partially torn off. Black smoke billowed from the site of the explosion on a palm tree-lined boulevard.
The blast came as Mursi appeared in court on Sunday on charges of conspiring with foreign groups to commit terrorist acts in Egypt.
Egypt has been rocked by political turmoil and violence since the downfall of Mursi, its first democratically-elected president. Islamist militants based in the Sinai have stepped up attacks on security forces, killing hundreds. The insurgency has also taken hold elsewhere, with attacks in towns and cities in the mainland, including Cairo.
"The militants in Sinai are now looking for soft targets, without entering confrontation with police and armed forces who have taken precautions," said Mustapha Kamel Al-Sayid, a professor of political science at Cairo University.
"This is a cheap win for them without a high risk."
While the army-installed government has succeeded in driving the Muslim Brotherhood underground with a security crackdown, it is struggling to cope with increasing pressure from militants in the Sinai, where several army offensives have failed to root them out.
The militants are scattered around a desert and mountain landscape they have mastered. Tight connections with bedouins and smugglers help them avoid capture.
The most prominent Sinai-based group, Ansar Bayt al-Maqdis, has claimed responsibility for a number of bombings, including an attempt to kill the interior minister in Cairo last year. The organisation also said it was behind a missile attack on a military helicopter last month that killed five soldiers.
Egyptian authorities have declared the Muslim Brotherhood a terrorist group and make no distinction between the Sinai militants and Mursi's supporters.
Sisi had hoped a political roadmap he unveiled after toppling Mursi would bring stability to Egypt, which has a peace treaty with Israel and contains the Suez Canal.
(Additional reporting by Maggie Fick; Writing by Michael Georgy; Editing by Janet Lawrence)