Iran has arrested five members of hotel staff after four Saudi Arabian pilgrims died and 20 more taken ill in a case of mass poisoning that appeared to be accidental, local news agencies reported on Monday.A group of 33 pilgrims from Saudi Arabia's Shi'ite Muslim minority was exposed to poison on Sunday while staying at a hotel in the north-eastern city of Mashhad, state news agency IRNA reported."An investigation has been launched into the management and staff of the hotel, and five people are under arrest," Gholamhossein Mohseni-Ejei, a judiciary spokesman, was quoted as saying by IRNA.He said that a total of 24 members of the Saudi group were admitted to hospital, of whom four had died. Some of the others were still in hospital and the rest had been discharged, he said.Rassoul Dinarvand, the head of Iran's Food and Drug Organization, was quoted by IRNA as saying those who died were a 14-year-old girl and three infants. He said the poison responsible was typically used for pest control but had been banned because of its danger to humans.A provincial justice official told the ISNA agency that those arrested were the hotel manager and four other members of staff. He ruled out foul play, blaming poor cleaning standards in the hotel rooms for the poisoning.Mashhad is home to the shrine of Imam Reza, a revered figure in Shi'ite Islam, and attracts millions of pilgrims each year from around the Muslim world. Saudi Arabia has a sizeable Shi'ite Muslim minority, which has tense relations with the Sunni political and religious establishment, some members of which accuse them of being more loyal to Iran than to the kingdom.The two countries are locked in a power struggle that has played out across the region and taken on a sectarian dimension. Each accuses the other of creating instability in the region.