Hillary Clinton has warned of 'tougher sanctions' against Iran's nuclear programme
US Secretary of State Hillary Clinton has warned that Iran faces "very tough sanctions" if it rejects offers of engagement over its nuclear programme.
The US was "laying the groundwork" for such measures if Iran refused dialogue or the process failed, Mrs Clinton told the House Foreign Affairs Committee.
At the same time, the US was more than willing to reach out to Iran, she said.
Iran says its nuclear programme is for peaceful purposes, but has been accused by critics of seeking nuclear weapons.
On Wednesday, Tehran said in a statement it was ready for "constructive dialogue" with world powers on the issue, but insisted it would continue with its nuclear activities.
Last week, President Mahmoud Ahmadinejad said he would present a new package for discussion, but has yet to give details.
Tehran has continued to enrich uranium despite lengthy negotiations in recent years with major powers and despite the imposition of sanctions by the UN Security Council.
Earlier this month, the six powers co-ordinating policy with Iran over its nuclear ambitions urged the Iranians to engage in fresh talks on a diplomatic solution.
And in a video message last month, US President Barack Obama offered "a new beginning" to the Iranian people and leaders, saying his administration was committed to diplomacy.
Speaking to US lawmakers on Wednesday, Mrs Clinton said the US was both pursuing a diplomatic path over Iran and laying the groundwork for possible sanctions.
She said that by engaging in diplomacy, the US would "gain credibility and influence with a number of nations who would have to participate in order to make the sanctions regime as tight and crippling as we would want it to be".
Iran says it is developing nuclear power for civilian purposes
Dealing with the threat posed by Iran and its nuclear programme was a priority, Mrs Clinton said, and the US was now a "full partner" in international talks on the issue.
She was testifying before the House for the first time since her confirmation hearings in January.
The BBC's state department correspondent, Kim Ghattas, in Washington says Mrs Clinton's message underscores the dual-track approach the Obama administration is taking to dealing with America's long-term foe.
The idea is to reach out, our correspondent says, but to prepare for the possibility these efforts may fail.
It is a carefully calibrated message, as the Obama administration does not want Tehran to feel there is a deadline, but equally it cannot engage in open-ended discussions with the Iranians.
Washington also wants to make sure it has the international community on its side, our correspondent says, so that if the approach fails the Obama administration can turn to Europe, but also Russia and China, to say they need now to support the US in seeking tougher action.
Under former President George W Bush, there were no direct US nuclear talks with Iran.
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