The United States Monday approved a waiver to ease sanctions on Iran, its part of an international deal after the Islamic republic began imposing curbs on its suspect nuclear program.

"Iran has begun to take concrete and verifiable steps to halt its nuclear program," State Department spokeswoman Jen Psaki said, adding it was "an unprecedented opportunity" to resolve global concerns over the atomic program.

US Secretary of State John Kerry has approved the waiver, which will allow limited relief to help Iran's crippled economy, and it would be sent to Congress on Monday, she added.

Earlier Monday, Iran announced it had halted production of 20 percent enriched uranium, marking the entry into force of the landmark interim deal with world powers.

White House spokesman Jay Carney hailed Iran's actions as "an important step forward."

"These actions represent the first time in nearly a decade that Iran has verifiably enacted measures to halt progress on its nuclear program, and roll it back in key respects," he said in a statement.

"Iran has also begun to provide the IAEA with increased transparency into the Iranian nuclear program, through more frequent and intrusive inspections and the expanded provision of information to the IAEA."

In return, the United States and the European Union each announced an easing to sanctions as negotiated with world powers under the six-month deal reached on November 24.

During the six months, Iran and the P5+1 group of powers, including the US, China, Russia, France, Britain and Germany, will attempt to negotiate a trickier long-term accord to end the nuclear standoff with Iran and avert a possible war.

In Washington, the US administration "has taken the necessary steps to pause efforts to further reduce Iranian crude oil exports," a separate US fact sheet said.

This will allow the six nations that buy Iranian oil to continue to do so for the duration of the deal.

Sanctions will also be suspended on non-Americans trading in Iran's petrochemical exports, as well as certain trade in gold and precious metals and Iran's automotive sector.

But senior US administration officials said the sanctions easing offered Monday was "modest financial relief."

"I would underscore that it does not mean that Iran is open for business. Quite on the contrary, the overwhelming majority of our sanctions, and the basic structure of oil and banking sanctions remain in place," a senior administration official said on a conference call.

"The administration is committed to aggressively enforcing those sanctions," he said.

Psaki warned too that the coming "negotiation to reach a comprehensive agreement that addresses all of the international community's concerns will be even more complex, and we go into it clear-eyed about the difficulties ahead."

"But today's events have made clear that we have an unprecedented opportunity to see if we can resolve this most pressing national security concern peacefully. That remains our goal, and that is our challenge ahead."