Meeting in Brussels on Friday, the six powers expressed disappointment that Iran has "not responded positively" to a deal to ship low-grade nuclear fuel abroad or agreed to new talks.

But US State Department deputy spokesman Robert Wood said that the United States and its negotiating partners Russia, China, Britain, France and Germany were not yet "at the point" of closing the window on dialogue.

Speaking to reporters in Washington, Wood said that the six powers would hold another meeting to discuss "next steps" but added that no date for a meeting had yet been determined.

In the past, the United States has raised the possibility of another round of sanctions against Iran.

"We're certainly hopeful that they will change their mind," Wood said when a reporter suggested that the hesitation to immediately discuss tough new measures indicated the six powers were holding out for a positive answer from Tehran.

The plan to ship low-enriched uranium abroad, under a UN-backed deal aimed at easing fears about what Iran is doing with its nuclear stockpile, "is something that the Iranians agreed to in principle," Wood reminded reporters.

In Geneva on October 1, Iran said it was ready to ship out most of its known low-enriched uranium to Russia for further enrichment. The material would then be turned into fuel for the Tehran research reactor by France.

"I don't know why Iran hasn't been able to say yes up until now. It could have to do with ... the internal political situation in Iran. But it's really hard to say," Wood said.

The Iranian leadership has been facing down domestic protests since Mahmoud Ahmadinejad was re-elected president in June in a widely-disputed election.

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