PITTSBURGH/VIENNA - U.S. President Barack Obama and other Western leaders accused Iran on Friday of building a secret nuclear fuel plant and demanded Tehran immediately halt what he called a direct challenge to the international community.

Obama went public with the charge in a joint appearance with British Prime Minister Gordon Brown and French President Nicolas Sarkozy at a Group of 20 summit in Pittsburgh, sharpening a standoff with Iran over its disputed nuclear program.

It is time for Iran to act immediately to restore the confidence of the international community by fulfilling its international obligations, Obama said, adding that Tehran had been building the plant in secret for years.

The U.N. nuclear watchdog said earlier on Friday that Iran had just told it of a second uranium enrichment plant under construction, a belated disclosure sure to heighten Western fears of an Iranian bid for nuclear weapons.

The International Atomic Energy Agency said Iran -- which insists its nuclear program is for peaceful purposes to generate electricity -- had disclosed the existence of the plant to IAEA Director-General Mohamed ElBaradei on Monday.

Obama accused Iran of breaking rules that all nations must follow and called for international inspectors to immediately investigate this disturbing information.

The disclosure, extending a history of Tehran withholding sensitive nuclear plans from U.N. nonproliferation inspectors, gave grist to Western calls to consider tougher U.N. sanctions against Iran ahead of October 1 talks with six world powers.

Sarkozy said Iran was taking the international community down a dangerous path and threatened new sanctions if Iranian leaders did not change course by December.

Brown said Iran's defiance should harden the resolve of the international community, which must now draw a line in the sand against Tehran.

While lodging a serious accusation against Tehran, Obama said: We remain committed to serious, meaningful engagement with Iran to address the nuclear issue through the P5+1 negotiations.

Since taking office in January, Obama has sought to engage Iran diplomatically but has been met mostly with defiance.

A U.S. official said the nascent plant was believed to be designed for about 3,000 centrifuges for enriching uranium.

Iran is under U.N. sanctions for refusing to suspend enrichment and denying access the IAEA needs to clarify Western intelligence indications that Iran has geared nuclear research to developing nuclear bombs, not generating electricity.

(Additional reporting by William Maclean in London and Matt Spetalnick in Pittsburgh; Editing by Howard Goller and John O'Callaghan)

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