The State Department reiterated Washington's support for efforts by its European allies to negotiate an end to Iran's nuclear weapons ambitions and said the ball was in Tehran's court.

"I think the issue is not what the US will or won't do. We've been over this ground before," deputy department spokesman Adam Ereli told reporters.

"The issue is, what actions will Iran take or won't Iran take to meet the concerns of the international community, to answer the questions that have been posed to it, and to bring itself into line with accepted international practice."

Mohamed ElBaradei, head of the International Atomic Energy Agency (IAEA), said Monday Washington would have to give Iran regional security assurances if the European-led nuclear talks were to succeed.

ElBaradei said, "At an appropriate time the United States will have to be fully engaged" because "regional security (in the Middle East) is very much not only a European affair."

After expressing initial skepticism, the United States has backed the efforts by Britain, France and Germany to wean the Iranians off their suspected nuclear arms programs.

Washington agreed earlier this month to help out with trade incentives, such as lifting its objection to Iran's membership in the World Trade Organization.

But while vowing to pursue diplomatic efforts with Iran, Washington has refused to take the military option off the table.

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