US officials Wednesday slammed a campaign of "unrelenting repression" ahead of Iran's presidential elections, and said the outcome would be very hard to predict amid a secret vetting process.

The future direction of the next Iranian leadership in ongoing talks with world powers about the Islamic republic's suspect nuclear program was also difficult to determine, top State Department officials told US lawmakers.

"There are probably some candidates who would be perceived by us as more interested in looking at the nuclear negotiations in a more positive vein," under secretary of state Wendy Sherman said.

"However the nuclear file is held by the supreme leader and no one else, and he is the final decision maker regarding the nuclear file."

The race for Iran's highest elected office took a stunning turn at the weekend when former president Akbar Hashemi Rafsanjani and top nuclear negotiator Saeed Jalili officially registered for the June 14 election.

President Mahmoud Ahmadinejad is constitutionally barred from seeking a third consecutive term. But on Saturday he endorsed his controversial aide and ex-chief of staff Esfandiar Rahim Mashaei.

Jalili, a known hardliner, is close to Iranian supreme leader Ayatollah Ali Khamenei, and was Wednesday in Istanbul for a new round of talks with EU foreign policy chief Catherine Ashton.

But the final line-up of presidential candidates will not be known until later this month when Iran's Guardians Council releases the approved list of names after the vetting process.

Denouncing "a deliberate and unrelenting level of repression in the lead-up to these elections," Sherman told the Senate Foreign Relations committee that the council was "using vague criteria to eliminate potential candidates."

"Without a transparent process, it is difficult for us to say whether Iran's elections will be free, fair, or represent the will of the Iranian people."

Conservatives were "pushing very hard against Mashaei and Rafsanjani, because they don't see either of those candidates as tough enough," she said.

"We don't know the impact -- as you may recall in 2009, everyone thought Ahmadinejad was going to be one kind of leader and he's turned out to be quite another kind of leader."

Ahmadinejad's disputed re-election in 2009 sparked massive street protests, leading to a heavy-handed regime crackdown and the arrest of hundreds of journalists, activists and reformist supporters.

"We take no sides in Iran's presidential election," Sherman insisted.

"The decision about who leads Iran is for the Iranian people, who should have every opportunity to express freely and openly their opinions, ideas, and hopes for the future of their country."