The United States expects Iran to reduce its stockpile of low enriched uranium to the levels required under an interim nuclear deal by a June 30 deadline, the US State Department said on Wednesday.

"If they don't, that'll be a problem," State Department spokeswoman Marie Harf told reporters at her daily briefing, saying the Iranians had always gotten to the prescribed levels under interim nuclear deals. "We expect that they will."

Under an interim nuclear pact struck in November of 2013 and renewed the following year, every six months Iran must reduce its stockpile of low enriched uranium (LEU) that is enriched to a purity of up to five per cent, to a maximum of about 7,650 kg.

According to the latest report of the U.N. nuclear watchdog, the Vienna-based International Atomic Energy Agency, Iran has about 8,714.7 kg of low enriched uranium. If refined much further, enriched uranium can provide fuel for nuclear weapons.

Harf said Iran's LEU stockpile had gone up and down and but that it had always gotten back to the 7,650 level by previous deadlines, as required. Under a subsequent agreement struck on April 2, Iran must eventually cut the stockpile to 300 kg.

The New York Times, which on Tuesday reported on Iran's having increased, rather than decreased, its stockpile of LEU in recent months, said the rise posed a major diplomatic and political challenge to the United States.

Harf denied this was an obstacle in talks between Iran and six big powers, including the United States. "It's not," she said flatly.

The seven nations are seeking to negotiate a deal by a June 30 deadline under which Iran would curb its nuclear activities in exchange for relief from economic sanctions.

The United States and some of its allies suspect Iran is using its nuclear energy program as a cover to develop an atomic weapons capability. Iran denies this, saying its program is for solely peaceful purposes such as producing medical isotopes. – Reuters