The United States has called into question the credibility of Iran's presidential election next month, criticising the disqualification of candidates and accusing Tehran of disrupting Internet access.
On a visit to Israel, US Secretary of State John Kerry also warned that time was running out to resolve the deadlock over Iran's contested nuclear programme.
Iran's Guardian Council, the state body that vets all candidates, has barred a number of hopefuls from the roster in the June 14 ballot, including former president Akbar Hashemi Rafsanjani, who is seen as sympathetic to reform.
"The Council narrowed a list of almost seven hundred potential candidates down to... officials of their choice, based solely on who represents the regime's interests," Kerry said shortly before flying out of Israel.
"That is hardly an election by standards which most people in most countries judge free, fair, open, accessible, accountable elections."
Most of the remaining eight men left on the Iranian ballot are seen as loyalists to Supreme Leader Ayatollah Ali Khamenei.
Kerry, whose country's decades-old rift with Iran has widened over the latter's nuclear ambitions, said Washington saw "troubling signs" that the Iranian government was slowing down or cutting off Internet access to its citizens.
"Ultimately the Iranian people (will) be prevented not only from choosing someone who might have reflected their point of view, but also taking part in a way that is essential to any kind of legitimate democracy," he said.
Israel and most Western powers, including the United States, believe Iran is seeking to obtain nuclear weapons - something Tehran has denied.
Israel, which is assumed to have its own nuclear arsenal, has warned it might attack Iran if it does not halt its atomic work, but Kerry said he hoped a diplomatic solution could still be found.
"Our hope is, for the sake of the region, the world, the Iranian people, ourselves, that we can have a peaceful resolution, but it is going to have to be demonstrated much more affirmatively than it has been to date that Iran is interested in that kind of a solution," he said.
The International Atomic Energy Agency (IAEA) said this week in a report that Iran was trying to accelerate its uranium enrichment programme.
"The clock is clearly ticking," Kerry added.