The United States warned on Friday that diplomatic overtures from Iranian President Hassan Rowhani, while welcome, are not enough for it to consider loosening sanctions aimed at Tehran's nuclear program.
White House national security spokesman Ben Rhodes said President Barack Obama has no plans to meet Rowhani at the UN General Assembly next week, and would maintain pressure on Iran when he addresses the body.
Washington believes Iran is seeking to develop the capability to build a nuclear weapons and has marshalled an economic sanctions regime to force Tehran to negotiate a way to open its program to international inspection.
Tehran's Islamist government denies that it is developing a bomb, while insisting it has a right to explore civilian nuclear power and research, and since Rowhani's election in June has been making diplomatic gestures.
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French President Francois Hollande, another champion of tough sanctions, is to meet Rowhani at the General Assembly in New York, and there has been mounting speculation that Obama might engineer an brief encounter.
But Rhodes said Washington was still waiting for more concrete evidence that Iran is ready to make concessions on the nuclear dossier.
"We've always made clear that we'll make judgments based on the actions of the Iranian government not just on their words," he said.
"I note that there have been some positive developments in terms of prisoners and some of the comments made by President Rowhani, but those are clearly not sufficient in the eyes of the international community.
"We don't have a meeting scheduled with President Rowhani at the UN General Assembly."
Rhodes said the United States had made it clear "that we do have a preference for resolving this issue diplomatically" but warned: "We want to make clear that there's not an open-ended window for diplomacy."