The United States on Monday ratcheted up its efforts to isolate Iran for its suspected nuclear weapons program, targeting Tehran with currency and auto-sector sanctions.

U.S. President Barack Obama imposed sanctions on foreign financial institutions that conduct or facilitate significant transactions in the Iranian rial currency.

In an executive order, the president also approved sanctions against people who do business with Iran's auto sector, which the White House said was a major source of revenue for Tehran.

The United States and Western powers have imposed a series of economic sanctions aimed at pressuring Iran into halting what they say is a drive to build a nuclear weapon. Tehran says its nuclear program is purely for generating power and for medical devices.

Last week, the United States blacklisted eight companies in Iran's petrochemical industry.

"We hold the door open to a diplomatic solution that allows Iran to rejoin the community of nations if they meet their obligations," White House press secretary Jay Carney said in a statement. "However, Iran must understand that time is not unlimited. If the Iranian government continues down its current path, there should be no doubt that the United States and our partners will continue to impose increasing consequences."

The sanctions imposed on the rial on Monday included a ban on maintaining significant accounts outside Iran denominated in that currency. It is the first time that trade in the rial has been targeted directly for sanctions, the White House said.

Also Monday, International Atomic Energy Association Chief Yukiya Amano said nuclear investigators may no longer find anything if granted access to Iran's Parchin military site, in view of suspected Iranian efforts to remove any traces of illicit atomic activity there.

Amano also said his agency's talks with Iran on unblocking an IAEA inquiry into possible nuclear arms research by Tehran had been "going around in circles" for some time.

Amano was airing unusually blunt criticism that reflected the mounting tension over Iran's disputed nuclear energy program that has increased fears of a new Middle East war.

Israel, widely assumed to be the Middle East's only nuclear-armed state, sees Iran's declared civil nuclear program as the most serious risk to its security and has threatened air strikes if diplomacy and sanctions fail to rein in Tehran.

Amano, director general of the International Atomic Energy Agency, vented growing frustration at the lack of results in getting Iran to address international concerns. Tehran denies its nuclear energy quest is a disguised bid for atomic bombs.

In hard-hitting comments to the IAEA's 35-nation Board of Governors and later at a news conference, he also said Iranian advances in building a research reactor and in its uranium enrichment work were in "clear contravention" of UN Security Council resolutions calling for a suspension in such activities.

The IAEA has been trying since early 2012 to engage with Iran over what the Vienna-based UN agency calls the "possible military dimensions" to the country's nuclear program.

But 10 rounds of negotiations in the last 17 months have failed to achieve any breakthrough. Western diplomats accuse Iran of stonewalling the IAEA, an allegation Tehran rejects.