The resolution from the five permanent members of the UN Security Council and Germany "expresses deep and increasing concern about the unresolved issues regarding the Iranian nuclear program, including those which need to be clarified to exclude the existence of possible military dimensions."

The resolution, due to be discussed by the International Atomic Energy Agency's board of governors on Friday, which is likely to approve it, stops short of setting a deadline for Iran to address the agency's concerns, however.

Instead it "requests the Director General to include in his progress report to the March 2012 meeting of the Board of Governors an assessment of the implementation of this resolution."

It says it is "essential for Iran and the Agency to intensify their dialogue" and calls on Tehran "to comply fully and without delay with its obligations under relevant resolutions of the UN Security Council."

It expresses "continuing support for a diplomatic solution, and calls on Iran to engage seriously and without preconditions in talks aimed at restoring international confidence in the exclusively peaceful nature" of Tehran's nuclear programme.

Last week, the agency came the closest yet to accusing Iran outright of seeking to develop nuclear weapons, in a report immediately rejected by the Islamic republic as "baseless."

But the report laid bare deep differences within the so-called P5+1 bloc dealing with the Iran question, the five permanent members of the UN Security Council -- the United States, China, Russia, Britain, France -- and Germany.

Washington, Paris and London jumped on the IAEA report as justification to tighten the screws on Iran, already under four rounds of Security Council sanctions, and additional US and European Union restrictions.

But Beijing, which relies heavily on Iranian oil imports, and Moscow, which also has close commercial ties and built Iran's only nuclear power plant, have been far more cautious.

"This, in my judgment, is one of the few remaining actions short of an embargo of Iranian shipping and military intervention to slow or end the Iranian nuclear program," said Republican Senate Minority Leader Mitch McConnell.

The measure, crafted by Republican Senator Mark Kirk, would empower President Barack Obama to cut off any foreign financial institution that does business with Iran's central bank from the US economy and to freeze its US assets.

"Without immediate and serious action, the Islamic Republic of Iran will have a nuclear weapons capability in the near future," Kirk said, adding: "We must act now or face the consequences of a nuclear Iran."

Kirk said it was "quite likely" Iran would transfer any nuclear weapons to Islamist groups like Hezbollah and Hamas, and worried about the prospects for "a nuclear arms race in the Middle East, from Saudi Arabia to Egypt."

The amendment includes a six-month grace period for oil-related transactions "to ease the burden on US allies and send a calming signal to the oil markets," Kirk's office said.

It would exempt firms that engage in transactions with the central bank in connection with sales of food, medicine, or medical devices to Iran, which denies Western charges that it seeks nuclear weapons under the guise of a civilian energy program.

And it would give Obama limited ability to waive application of the sanctions if he certifies doing so is in the US national security interest.

Last week, the UN nuclear watchdog agency came the closest yet to accusing Iran outright of seeking to develop nuclear weapons, in a report immediately rejected by the Islamic republic as "baseless."

The evidence included a bus-sized steel container visible by satellite for explosives testing and weapons design work, including examining how to arm a Shahab-3 missile capable of reaching Israel with a nuclear warhead.

Despite heavy US and international sanctions already in force, "the Iranian regime has not been deterred from conducting activities relevant to the development of a nuclear explosive device," said McConnell.

"Iran remains undeterred, and the United States is left with fewer options for dealing with the Iranian nuclear program as time elapses," he said. "The time has come for our country to sanction the Central Bank of Iran."

US officials said Tuesday that Washington was weighing new sanctions aimed at Iran's central bank but seeking to avoid handing Tehran unintended gains or alienating young Iranians, US officials said Tuesday.

The officials told lawmakers the Obama administration was studying the proposal to ensure it does not result in a rise in the price of oil, which would boost Iran's cash reserves.

They also warned against steps that hurt Iran's people, which risk angering young Iranians who they said were inclined to favor the United States and could end up being agents of change in the Islamic republic.