France warned on Monday that already serious international concerns over Iran's nuclear programme have deepened after Tehran said it would start building a third uranium enrichment site next year.

"We expect Iran to comply with its international obligations," said foreign ministry spokeswoman Christine Fages, after Iran defied the world powers that recently slapped sanctions on Tehran over its nuclear work.

"This announcement only worsens the international community's serious concerns about Iran's nuclear programme," she added in a statement.

Iran's atomic chief Ali Akbar Salehi said on Sunday that the Islamic republic's search for sites for 10 new enrichment facilities "is in its final stages."

He said the construction of one of these will begin by the end of the current Iranian year, that is before March 2011, or the start of the next year.

Iran is already enriching uranium at its main plant in the central city of Natanz and is building a second enrichment facility inside a mountain at Fordo, southwest of Tehran.

The country's hardline president, Mahmoud Ahmadinejad, announced the planned construction of 10 new enrichment plants late in 2009 after Tehran was censured by the UN atomic watchdog over building the Fordo facility.

Salehi, who is also one of 12 vice presidents of Iran, has previously said that any new uranium plants the Islamic republic builds would be located at places which cannot be targeted by air strikes.

He did not specify where the third plant would be located.

"If confirmed, this decision would be a fresh violation by Iran of its obligations under six United Nations Security Council resolutions," Fages said.

The most recent resolution, dating from June 9, says "without ambiguity" that Iran must not undertake "the construction of any new enrichment facility," the spokeswoman added.

France considers that "the enrichment programme has no identifiable civilian end" as Iran's only reactor likely to be operational in coming years is at Bushehr, a power plant for which Russia will supply fuel.

Russian atomic agency Rosatom has said that the highly sensitive Bushehr plant in southern Iran will be formally launched on August 21.

Iran's arch-foes the United States and Israel have never ruled out military strikes against Tehran to halt its nuclear programme which they suspect is aimed at making weapons.

Tehran denies this, saying its atomic programme has purely peaceful goals.

Enriched uranium can be used as fuel to power nuclear reactors as well as to make the fissile core of an atom bomb.