President Mahmoud Ahmadinejad takes a tour of the Isfahan nuclear facility
Iran's president has inaugurated the country's first nuclear fuel production plant, making what correspondents say was a defiant speech.
President Mahmoud Ahmadinejad said Iran was open to an offer of fresh talks with world powers, but only if they were based on "justice" and "respect".
Once operational, the new plant could produce sufficient plutonium for two nuclear weapons a year, analysts say.
Iran denies Western claims that it aims to build a nuclear bomb.
The BBC's Jon Leyne in Tehran says Mr Ahmadinejad made it clear he did not see any change in US policy towards Iran, and therefore offered nothing in return.
"The Iranian nation has from the beginning been after... negotiations based on justice and complete respect for rights and regulations," Mr Ahmadinejad said.
"One-sided negotiations, conditional negotiations, negotiations in an atmosphere of threat are not something that any free person would accept," he said.
Reacting to Mr Ahmadinejad's comments, US state department spokesman Robert Wood said the ball was still in Iran's court with regards to direct talks.
"We want to engage Iran and we... have said so very clearly and publicly, and so we wait for Iran to reciprocate," said Mr Wood.
Mr Ahmadinejad said Iran had tested two new types of centrifuge with higher capacities at a uranium enrichment plant in Natanz.
Meanwhile, nuclear chief Gholam Reza Aghazadeh said Iran has increased the number of centrifuges it is running at the plant to 7,000 from 6,000 in February.
If verified, that would be a significant step forward, Jon Leyne reports, though the last report from international inspectors said Iran had less than 4,000 working centrifuges.
US Secretary of State Hillary Clinton cast doubt on Iranian claims of major progress.
"We don't know what to believe about the Iranian programme. We've heard many different assessments and claims over a number of years," she said at a news conference in Washington.
The inauguration of the new plant in Isfahan shifts attention to a parallel programme to Iran's uranium enrichment activities, BBC diplomatic correspondent Jonathan Marcus reports.
Once it is fully operational it could produce sufficient plutonium for two nuclear weapons a year, should Iran choose to separate the plutonium from the reactor's spent fuel, he says.
The newly-opened plant will produce pellets of uranium oxide that could be used to fuel a heavy-water reactor in Arak expected to be completed some time between 2011 and 2013.
It signals Iran has reached the final stage in the nuclear fuel cycle, analysts say.
The move came as Tehran considered an offer by six world powers, including the United States, to take part in fresh talks on the crisis.
Russia and China - trading partners of Iran - have urged Tehran to accept the invitation.