Iranian President Mohammad Khatami has said Tehran will never give up nuclear technology, as international pressure on his government continues to mount.
The US believes Iran is getting closer to developing nuclear arms
He warned of "massive" consequences if Iran was treated unfairly.
Mr Khatami said again that the nuclear programme was peaceful and needed to produce power, rejecting US suspicions that it is a cover for weapons.
EU powers want Tehran to end uranium enrichment - a key part of nuclear arms production - permanently.
"We give our guarantee that we will not produce nuclear weapons because we're against them and do not believe they are a source of power," Mr Khatami told foreign ambassadors in Tehran.
"But we will not give up peaceful nuclear technology," he added.
In the face of international pressure, the Iranian government has called for large numbers of its supporters to go out onto the streets on Thursday to mark the anniversary of the 1979 Islamic revolution.
'Our clear right'
In Washington, President George W Bush said a nuclear-armed Iran would be "a very destabilising force in the world" and urged the West to work together to stop such an outcome.
The message was reinforced by US Secretary of State Condoleezza Rice during a tour of Europe this week.
She said Washington had no deadline to refer the issue to the United Nations Security Council for possible sanctions, adding that diplomacy had to be given every chance to work.
While talks with Germany, France and Britain continue in Geneva, Iran has suspended uranium enrichment, which can be used to make weapons-grade fuel.
But in his speech Mr Khatami said that enrichment was "our clear right" and that Iran had suspended it only "to show our goodwill".
He added: "If we feel others are not meeting their promises, under no circumstances would we be committed to continue fulfilling ours.
"And we will adopt a new policy, the consequences of which are massive and would be the responsibility of those who broke their commitments."
The European countries would like to use a package of incentives to induce Iran to give up its nuclear ambitions, but Tehran has said it is disappointed with what is on offer so far.
It says it can only continue talks for a matter of months, not years.
Enriched uranium can be used to produce nuclear power, but the technology behind it can also be used to develop weapons-grade nuclear material.