Welcoming a rare consensus on Iran among the globe's most powerful nations at the L'Aquila summit, Obama told reporters he hoped Tehran would recognize that "world opinion is very clear".

The G8 joint declaration expressed "serious concern" over post-election violence in Iran but called for a negotiated resolution to the standoff over Tehran's nuclear programme, giving it until September's G20 summit.

"And that's been always our premise, is that we provide that door," said Obama.

"But we also say we're not going to just wait indefinitely and allow for the development of the nuclear weapon, the breach of international treaties, and wake up one day and find ourselves in a much worse situation and unable to act.

"So my hope is that the Iranian leadership will look at the statement coming out of the G8 and recognize that world opinion is clear."

Russia, Iran's closest thing to an ally at the world's most powerful top table, argued that the bloody crackdown on opposition protestors after the June 12 polls was an "internal matter", and Washington failed to find support for its call for tougher sanctions against Tehran.

But after his first G8 summit, Obama played down the lack of concrete steps and talked up a united front, particularly since he had avoided a row with Russia.

"What we wanted was exactly what we got, which is a statement of unity and strong condemnation about the appalling treatment of peaceful protestors," he said.

"And so I think the real story here was consensus in that statement, including Russia. Which, you know, doesn't make statements like that lightly."

Obama pronounced himself pleased with the second part of the declaration, "that we will reevaluate Iran's posture towards negotiating the nuclear weapons policy," at the G20, to be held in Pittsburgh, Pennsylvania, ahead of the United Nations General Assembly.

"And I think what that does is it provides a time frame. The international community has said: 'Here's a door, you can walk through. That allows you to lessen tensions and more fully join the international community'."

"If Iran chooses not to walk through that door, then you have on record the G8 to begin with, but I think potentially a lot of other countries that are going to say, we need to take further steps."

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