By Marcus George and Yeganeh Torbati DUBAI (Reuters) - Few Iranian presidential elections have been so unpredictable but the next few days will at least narrow down who will stand in the ballot on June 14, for which candidate registration starts on Tuesday and ends on Saturday.

Iran began Tuesday a five-day registration period for candidates in a June 14 presidential election, with a string of conservative hopefuls in the running but key reformists yet to come forward, reports said. The interior ministry started registering candidates from 8:00 am (0330 GMT), according to the media reports. Interior Minister Mostafa Mohammad Najjar advised hopefuls against waiting until the last day to register, while warning against early campaigning, said the state broadcaster's website.

An Iranian diplomat with ties to the reformist camp has been arrested following "a misunderstanding", Foreign Minister Alki Akbar Salehi was quoted as saying Thursday by the Isna news agency. Bagher Asadi's detention comes weeks ahead of the June 14 presidential election, the first since the wave of protests that followed the 2009 poll that saw Mahmoud Ahmadinejad defeat reformist candidates. "We hope that the misunderstanding over Mr. Asadi, who is a senior and respected employee of the foreign ministry, will be resolved and that he will be freed," Salehi said.

Intelligence Minister Heydar Moslehi on Thursday warned ex-presidents Akbar Hashemi Rafsanjani and Mohammad Khatami, without naming them, over their alleged role in the protest movement that followed Iran's disputed 2009 elections. The two former presidents have not yet announced if they will contest the June 14 presidential election to replace Mahmoud Ahmadinejad, who cannot stand for a third consecutive term.