Tehran - A bomb exploded on Wednesday in Zahedan in south-eastern Iran, killing at least 18 people, ISNA news agency reported. The bomb was planted in a car and went off in the early morning hours in a street in Zahedan just after a bus belonging the paramilitary revolutionary guards passed by. It was not immediately clear whether the victims were revolutionary guards or ordinary citizens. A guard spokesman told ISNA that one suspect has already been arrested but gave no further details. The Fars news agency quoted eye-witnesses as saying that a car stopped in front of the IRGC bus under the pretext of having engine problems. Right afterwards all four men in the car left the car and escaped the scene by motor-bikes previously parked there just before the planted bomb went off. The IRGC members in the bus, believed to be the main victims of the bombing, were all working in the administration of the IRGC office in Zahedan. As Zahedan, located in the Sistan-Beluchistan province bordering Afghanistan and Pakistan, is a centre for drug-trafficking, Fars speculated that a drug gang was involved in the bombing. A large number of IRGC anti-narcotic forces are stationed in the Sistan-Beluchistan province and frequently have clashes with not only local but also Afghan and Pakistani drug traffickers who try to smuggle the drugs via Iran to European markets. - Deutsche Presse-Agentur