SANAA (AFP) - Four South Korean tourists were killed and four more wounded by an explosion in the historic city of Shibam in southeast Yemen on Sunday, a security official in Hadramawt province said. "According to a preliminary toll, the explosion killed four South Korean tourists and wounded four more, as well as a Yemeni man," the official told AFP by telephone from Shibam. "We think the blast was caused by a bomb that had been planted there," the official added, speaking on condition of anonymity. Earlier a local official said the explosion "may have been a suicide attack, but this has not yet been confirmed." The security official said the blast came as the tourists were photographing the sunset, and that an inquiry had been launched. "The explosion happened as they were gathered on a hill called Khazzan that overlooks the city. They were on foot and taking pictures of the buildings in Shibam at the moment the sun went down," he added. Shibam, some 800km southeast of Sanaa, is a UNESCO world heritage site well known for its high-rise mud-brick buildings. "Its impressive tower-like structures rise out of the cliff and have given the city the nickname of 'the Manhattan of the desert'," the UNESCO World Heritage website says.