REPUBLIKA.CO.ID, VIENA - Iran and world powers locked horns on Wednesday over the future of a planned Iranian nuclear reactor that could yield plutonium for bombs as the United States warned "hard work" will be needed to overcome differences when the sides reconvene in April.

"We had substantive and useful discussions, covering a set of issues, including enrichment, the Arak reactor, civil nuclear cooperation and sanctions," European Union foreign policy chief Catherine Ashton told reporters after the two-day session.

The United States has called on Iran to scrap or radically alter the as yet-uncompleted reactor, but Tehran has so far rejected that idea while hinting it could modify the plant.

"We shared with Iran ideas that we have," a senior US administration official told reporters condition of anonymity. "We have long said that we believe that Arak should not be a heavy water reactor as it is, that we did not think that that met the objectives of this negotiation."

Enrichment is also a sticking point in the talks. "It's a gap (on enrichment) that's going to take some hard work to get to a place where we can find agreement," the US official said.

Enriched uranium can serve as fuel for nuclear power plants or, if refined to a high degree, for the core of an atom bomb.

The meeting in Vienna was the second in a series that the six nations - the United States, China, Russia, Germany, France, Britain - hope will produce a verifiable settlement on the scope of Iran's nuclear program, ensuring it is oriented to peaceful ends only, and put to rest the risk of a new Middle East war.

This week the two sides endeavored to iron out their positions on two of the thorniest issues: the level of uranium enrichment conducted in Iran, and its Arak heavy-water reactor that the West sees as a possible source of plutonium for bombs.

They appeared to reach no agreements and said only that they would meet again on April 7-9, also in the Austrian capital. However, Tehran's foreign minister voiced optimism that their July 20 deadline for a final agreement was within reach.

The broad goal is to transcend ingrained mutual mistrust and give the West confidence that Iran will not be able to produce atomic bombs while Tehran - in return - would win full relief from economic sanctions hamstringing the OPEC state's economy.

Iran denies that its declared civilian atomic energy program is a front for developing the means to make nuclear weapons, but its restrictions on UN inspections and Western intelligence about bomb-relevant research have raised concerns.

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