TEHRAN - Iran will start work on a new uranium enrichment nuclear plant, a senior official said on Monday, part of a big expansion of its nuclear program which has contributed to fears in the West it aims to build a bomb.

Defying Western pressure to curb its sensitive nuclear work, Iran announced in November it planned to expand its enrichment activities by building 10 new sites. The announcement was condemned by the United states and its European allies.

"The president has confirmed the designated location of a new nuclear site and on his order the building process will begin," Mojtaba Samareh-Hashemi, a senior adviser to President Mahmoud Ahmadinejad, told the semi-official ILNA news agency.

"New locations on which the plants should be constructed this year have been determined and the construction will start stage by stage," Samareh-Hashemi was quoted as saying.

Iran's top nuclear official Akbar Salehi told Reuters in February that Iran would start construction of two enrichment sites by March 2011.

Washington is pushing for a fourth round of United Nations sanctions on Iran in the coming weeks to pressure it to halt its enrichment-related work, which Tehran says is entirely peaceful.

Iran started higher-level enrichment in February, saying it needed the 20 percent enriched fuel for a research reactor in Tehran making medical isotopes. Such potent material is not necessary to generate electricity.

Tehran has said it is still willing to swap low-level enriched uranium for higher-grade fuel enriched abroad -- a move which would help address fears about Iran's enrichment activities -- but the exchange must happen on Iranian soil.

The West believed it had persuaded Iran, at talks in Geneva last October, to hand over some of its uranium stocks to be enriched abroad, but that deal fell apart soon afterwards.

Samareh-Hashemi said any import of enriched uranium would not mean Iran planned to stop its own enrichment.

"The domestic production of (nuclear) fuel does not contradict importing it," he said.

"We have started to produce uranium domestically based on our need to provide fuel for the Tehran research reactor and this will continue until our needs are met."

In a separate development, state-owned Jam-e-Jam daily said Iran's ambassador to the U.N. International Atomic Energy Agency, Ali Asghar Soltanieh, would soon be stepping down as his term was coming to an end. Soltanieh was not immediately available to comment.