US Secretary of State John Kerry expressed fresh doubt that Iran's presidential elections would change Tehran's controversial nuclear policy.
"I do not have high expectations that the election is going to change the fundamental calculus of Iran," Kerry told reporters.
"So we will continue to pursue... every effort to have a peaceful resolution, but Iran needs to understand that the clock is ticking."
Speaking in Tel Aviv last week, Kerry denounced the "lack of transparency" in the run-up to the June 14 vote, saying it was "highly unlikely that that slate of candidates is either going to represent the broad will of the Iranian people or represent a change."
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The world's major powers and Israel, which has threatened to conduct a preventive strike, accuse the Islamic republic of seeking to develop a nuclear weapon under the cover of a civilian energy program for allegedly peaceful purposes.
Iran vehemently denies the charge.
Washington and its P5+1 allies -- Britain, China, France, Germany and Russia, plus Germany -- have been negotiating for years with Tehran to find a solution to the diplomatic crisis.
Despite the bloc's quest for a peaceful resolution, "every month that goes by gets more dangerous," Kerry warned.
"The reality is that Israel would do what it needs to do to defend itself.
"The United States of America and others, including Russia and China and the global community have already spoken that it is unacceptable for Iran to unilaterally or any other way to have a nuclear weapon," he added.