The news comes just days ahead of the UN Security Council’s August 31 deadline for Iran to freeze the sensitive work or face the risk of sanctions.
Iran has maintained it has no intention to build nuclear weapons insisting its program aims only to generate electricity.
Iranian president, Mahmoud Ahmadinejad, again took a tough tone over the nuclear issue, and said his country's decision to pursue peaceful nuclear technology was irreversible.
"The Iranian people will pursue their path and will not forswear it under threats or force," Mr Ahmadinejad said late Sunday.
Iran’s lead negotiator, Ali Larijani, reinforced the president’s comments by telling state radio, "production of nuclear fuel is one of Iran's strategic objectives.”
“Any action to limit or deprive Iran could not force Iran to give up this goal, " Mr Larijani said.
However, the Islamic regime said it still sought talks on Western concerns about its nuclear programme.
"Iran is ready to hold discussions with the foreign ministers of the five permanent Security Council members and Germany, wherever and whenever," Mr Larijani said.
The UN Security Council has demanded that Iran suspend all uranium enrichment amid US-led concerns that its nuclear programme is cover for an attempt to produce an atomic bomb.
Mr Annan’s visit would come two days after the UN’s August 31 deadline.
Plant inauguration
On Saturday, Mr Ahmadinejad defiantly inaugurated a plant to produce heavy water for use in a new research reactor in Arak, central Iran.
Heavy water reactors can be used to produce plutonium, which is the fissile core of most modern nuclear weapons.
The White House responded with restraint at the news, and said merely that the United States is consulting with its partners about what to do next.
Meanwhile on Sunday, the deputy head of the Iranian Atomic Energy Organisation said Tehran planned to build a new light water reactor to produce electricity.
Missile test
At the same time, in a fresh show of its military might, Iran test-fired a long range radar-evading sea-to-air missile during war games that aimed to demonstrate readiness for "any threat".
The Thaqeb is Iran's first missile that is fired from underwater but flies above the surface to hit its target, distinguishing it from a torpedo. A brief video showed the Thaqeb exiting the water
and hitting a target less than 1.6 km away.
The test took place during large-scale military exercises that Iran has been holding since August 19. It was the latest in a series of new naval weapons Iran has unveiled this year to tout what it calls its new technological prowess in arms production.
Incentive package offered
A package of incentives backed by the six major powers: Britain, China, France, Germany, Russia and the United States, is dependent on the regime first agreeing to suspend enrichment.
But deputy foreign minister Mohammad Reza Bagheri insisted during a visit to key regional ally Syria that a formal moratorium on enrichment was out of the question.
"While cooperating with international institutions, we consider the suspension of enrichment as our red line," Mr Bagheri was quoted as saying in Damascus by Iran's official news agency IRNA.