MOSCOW - World powers began two days of talks with Iran on Monday to try to end a decade-long stand-off over Tehran's nuclear program and avert the threat of a new war in the Middle East.

Experts and diplomats say a breakthrough is unlikely, with Iran expected to demand recognition of its right to enrich uranium for what it says is a purely peaceful nuclear program.

The sides are no closer to agreement despite two rounds of negotiations since diplomacy over Iran's atomic program resumed in April after a 15-month hiatus.

Israel has threatened to bomb Iran if no solution to the dispute is found, oil markets are nervous over the prospect of intensifying regional tensions and the frail world economy can ill afford a further increase in oil prices.

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The nuclear-armed United States, Russia, China, France and Britain - plus Germany - will push Tehran over its enrichment of uranium to 20 percent fissile purity.

Such production represents a big technological advance towards making weapons-grade material.

The world powers are wary of letting diplomacy drag on without clear progress and giving Iran time to build up a program which they fear is aimed at developing weapons, although Tehran denies this.

"If Iran remains unwilling to take the opportunities these talks present, it will face continuing and intensified pressure and isolation," a Western official said.

That would mean more sanctions from the West, although not the United Nations Security Council because veto-holding China and Russia oppose further punitive measures.

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