The Obama administration rebuffed a call for a US-Iranian summit, but saw signs that growing international sanctions may force Iran into holding big-power talks about its nuclear aims.

The US Treasury Department at the same time imposed sanctions on 21 firms it believes are front companies for the Iranian government, the latest in a flurry of punitive steps taken by the international community since June.

White House spokesman Robert Gibbs rebuffed a proposal from Irans hardline President Mahmoud Ahmadinejad for face-to-face summit talks with his US counterpart Barack Obama about what he called "global" issues.

"We have always said that wed be willing to sit down and discuss Irans illicit nuclear program, if Iran is serious about doing that," Gibbs told reporters. "To date, that seriousness has not been there."

On Monday, Ahmadinejad criticized Obama for missing "historic opportunities" to repair the broken relations with Iran, which has had no diplomatic ties with Washington for three decades, and offered to meet him.

"We are hopefully coming for the UN assembly," Ahmadinejad said in an address to expatriate Iranians which was broadcast live on state television.

"We are ready to sit down with Mr. Obama face-to-face and put the global issues on the table, man-to-man, freely, and in front of the media and see whose solutions are better. We think this is a better approach."

Ahmadinejad, who sees the Arab-Israeli conflict and other regional problems as among important global issues requiring Iranian diplomacy, is expected to travel to New York for the UN General Assembly meeting next month.

US State Department spokesman Philip Crowley said Iran may now be seeking a dialogue with Washington because it is feeling the bite of sanctions imposed by the UN Security Council, the United States, European Union and others.

"The cost of doing business for Iran is going up," Crowley told reporters.

"Were encouraged by what were seeing... We sense that there may well be a willingness on the part of Iran to enter into the kind of dialogue that we have long sought," he added.

Apart from calling for a summit, Iran said Friday it was ready for immediate talks with the United States, Russia and France over a confidence-building exchange of nuclear fuel for a medical research reactor in Tehran.

Iran balked at the exchange involving France and Russia before it proposed in May a similar trade with Turkey, under a plan brokered by Brazil and Turkey that raised doubts in Washington.

Crowley added: "Were willing to meet Iran any time, any place within the P5-plus-1 to begin to address a series of issues, most significant to us, the nuclear issue."

He was referring to the permanent five UN Security Council members - the United States, Russia, China, Britain and France - plus Germany, which have been leading the diplomatic efforts to curb Irans nuclear ambitions.

Washington and other western nations suspect Irans uranium enrichment program masks a drive for atomic weapons. Iran denies the charge, saying it is pursuing the peaceful use of nuclear energy.

Citing Irans failure to comply with international demands to halt uranium enrichment, the United States has led the drive for sanctions, including a fourth round of UN Security Council penalties imposed in June.

Since then, the European Union, Australia and Canada have followed Washingtons example of moving tightly to enforce the UN sanctions as well as impose additional punitive measures of their own.

Japan on Tuesday imposed sanctions against Iran in line with the June UN resolution and said it plans to announce additional measures later this month.