WASHINGTON (AFP) - United States Secretary of State John Kerry on Monday approved a waiver to ease sanctions on Iran, after the Islamic republic began to impose curbs on its suspect nuclear programme.

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

Mr 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.

As agreed with world powers under a six-month deal reached on Nov 24, 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 which 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.

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 programme, 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 programme, through more frequent and intrusive inspections and the expanded provision of information to the IAEA."

Ms Psaki agreed "these actions today are significant steps in our efforts to achieve a diplomatic solution to prevent Iran from obtaining a nuclear weapon".

"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," she said.

"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."