Clinton says Iran engagement has expiry date

US calls for direct negotiations with Tehran grow louder with Secretary of State Hillary Clinton urging Iranian officials not to miss their cue.

After weeks on the sidelines, Clinton re-emerged on Wednesday to give a salvo of warnings and condemnations to the Tehran government in a major foreign policy speech.

Clinton said the Obama administration would like to move forward with plans to engage Iranian officials in face-to-face negotiations in the near future.

She, however, warned that the clock was ticking for Iran to accept White House’s offer, saying the bid will not remain open indefinitely.

"We ... understand the importance of trying to engage Iran and offering its leaders a clear choice — whether to join the international community as a responsible member or to continue down a path to further isolation," said Clinton.

"Direct talks provide the best vehicle for presenting and explaining that choice," she added.

Clinton said that both she and the US President Barack Obama believe that direct dialogue with Iran would guarantee success.

She went on to accuse the Iranian authorities of using ’deplorable and unacceptable’ actions to quell the post-election street violence that killed scores of Iranians in the past month.

For months, political heavyweights in Washington have claimed to be willing to engage the Tehran government on its nuclear case, but have yet to make any significant move to prove the sincerity of their plans.

Western powers accuse Tehran of secretly enriching weapons-grade uranium - an allegation Iran has repeatedly denied. Tehran has asserted that its uranium enrichment is a peaceful drive to produce electricity.

Incoming IAEA Chief, Yukiya Amano, also said on Friday that there is no conclusive evidence to prove that Tehran is enriching weapons-grade uranium.

Washington resumed calls for diplomacy and dialogue after Vice President Joe Biden openly suggested that the US would not stand in the way of an Israeli attack on Iran.

"Israel can determine for itself ... what’s in their interest and what they decide to do relative to Iran and anyone else," Biden said. "We cannot dictate to another sovereign nation what they can and cannot do when they make a determination --- if they make a determination — that they’re existentially threatened."

The remarks were widely interpreted as a long-sought green-light by the US for Israel to bomb Iranian nuclear sites.

Barack Obama made a quick attempt for damage control, explaining that his administration only seeks to resolve the Iranian nuclear issue "through diplomatic channels."