Diplomats from several nations meeting with Iran in Moscow depict the talks as significant. They say it could be the last in a series and if negotiators fail to make headway in persuading Tehran to stop higher-grade uranium enrichment, it's unclear if or when new talks would occur.

While Iran wants the other side to recognize its right to enrich and blink first by easing sanctions, the six nations say the onus is on Tehran to show it is ready to compromise.

Because it lives off its oil sales, Iran would be most immediately hurt by a lack of progress in Moscow followed by any long hiatus in new negotiations. But the White House also stands to lose.

Failed talks at Moscow with no immediate prospect of new meetings would expose President Barack Obama to criticism of weakness in dealing with Iran from Mitt Romney, his U.S. Republican presidential challenger, and from Israel, which has threatened to attack the Islamic Republic's nuclear installations should diplomacy fail.