The move comes amid growing Iranian frustration with the nuclear deal, with which UN inspectors say Tehran is complying but which has not led to a promised economic boom, with the Trump administration instead imposing sweeping sanctions.

Under the 2015 deal, which was negotiated under former president Barack Obama and still enjoys strong support among European powers, Iran was limited to keeping 300 kilograms of uranium enriched up to 3.67 percent -- far below the level needed to build nuclear weapons.

As part of the agreement, Iran was to sell any enriched uranium above that threshold on international markets in return for natural uranium, with Russia a key player.

But in Friday's policy change, Secretary of State Mike Pompeo said the United States would start to impose sanctions on anyone involved in the trade of natural for enriched uranium -- as well as in the storage of Iranian heavy water that was in excess of limits.

"The Trump administration continues to hold the Iranian regime accountable for activities that threaten the region's stability and harm the Iranian people. This includes denying Iran any pathway to a nuclear weapon," State Department spokeswoman Morgan Ortagus said.

At the same time, the United States issued new three-month waivers to allow the continuation of the major parts of the nuclear deal.

In particular, the United States said it will not target the transfer of scrap and spent nuclear fuel out of Iran and will not impose sanctions over work at Bushehr, the country's only civilian nuclear power station, although it said it may punish any activity that expands the site.

The Trump administration pulled out of the nuclear accord a year ago and instead vowed "maximum pressure" aimed at curbing the regional role of Iran, an arch-rival of US allies Saudi Arabia and Israel.

Some observers believe that the Trump team is trying to goad Iran into breaking the terms of the nuclear deal, thereby causing it to collapse and significantly deepening Tehran's international isolation.