Looking to heap economic pressure on Iran over its suspect nuclear program, US senators on Thursday introduced legislation aimed at collapsing the country's central bank with tough new sanctions.

"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.

The measure's prospects of becoming law seemed solid: 92 senators signed a letter earlier this year embracing the proposal, and Democratic Representative Howard Berman has introduced a companion piece in the House of Representatives.

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 told lawmakers Tuesday that Washington was weighing new sanctions aimed at Iran's central bank but sought to avoid handing Tehran unintended gains -- such as boosting the price of oil, which would swell its cash reserves.