The United States on Monday renewed its fears about Iran buying equipment for its nuclear program, amid reports it believes Tehran may be stepping up efforts to finish a suspect reactor.

Just over two weeks after global powers failed to secure a full deal to rein in Iran's nuclear program, the online Foreign Policy magazine revealed that the US had privately told a UN panel of experts monitoring sanctions that Iranian agents sought illicitly to obtain parts for the Arak heavy water nuclear complex.

"It's not breaking news that we are concerned about Iran's procurement activities," State Department spokeswoman Jen Psaki said.

But she refused to outline to reporters exactly what kind of acquisitions were raising eyebrows in Washington.

She insisted instead that "Iran has kept all of their commitments" made as part of an interim deal reached in November 2013.

Under that accord, Iranian leaders agreed to freeze parts of their nuclear enrichment program in return for the release of billions of dollars in frozen oil revenues.

Despite a further year of negotiations, the group known as the P5+1 which includes the United States failed to seal a full deal by a November 24 deadline saying gaps remained.

Now they have extended the deadline for talks until June 30, although US Secretary of State John Kerry said Sunday they hoped to pin down the final details in three to four months insisting progress had been made.

Foreign Policy said the US allegations were laid out in a November 7 report by a panel of experts that advises the UN Security Council committee which oversees compliance with UN sanctions on Iran.

An unnamed delegation told the panel it had seen "an increase in procurement on behalf of the IR-40 Heavy Water Research Reactor at Arak."

Diplomatic sources confirmed to Foreign Policy that it was the US who had presented the report.

The report could add fuel to Republican moves in the Congress to try to slap further sanctions on Iran, which the administration is trying to head off to give time for diplomacy to work.

"The wheels seem to be coming off of the administration's Iran strategy," said Representative Ed Royce in a statement, adding Tehran "is proving to be a determined cheater, showing no willingness to accept an effective verification regime."

"There are now multiple reports that Iran has violated its commitment to freeze its nuclear program."

Iran's nuclear chief last month ruled out further negotiations on the design of the Arak reactor that the West fears could be used to produce plutonium for an atomic bomb.

The future of the reactor is one of the main issues in the nuclear talks, with Iran insisting the unfinished reactor, located 240 kilometers (145 miles) southwest of Tehran, is solely for research purposes.

The United States has proposed transforming Arak into a light water reactor so that it produces far less plutonium, but Tehran has refused.