"At this point, I can't confirm any of the technical details. There is a lot that goes into a technical assessment of where the Iranians might be in their capability in operating a centrifuge cascade, whether it be a small one or a large one ... I couldn't offer an assessment for you as to where they stand in that process," State Department spokesman Sean McCormack told a news briefing.

However, White House spokesman Scott McClellan said in the day that Iran is "moving in the wrong direction" after the Islamic republic announced it has successfully enriched uranium for the first time.

Gholamreza Aghazadeh, Iran's Atomic Energy Organization said Tuesday that Iran had enriched uranium to a level used in nuclear power plants. The pilot enrichment plant in Natanz, south of Tehran, started working on Monday, he said.

Iranian President Mahmoud Ahmadinejad declared earlier "Iran will soon join the club of countries that have nuclear technology".

The United States has accused Iran of trying to develop nuclear weapons under the cover of civilian nuclear programs. However, Iran has said its nuclear programs are for civil purposes only.