The United States on Thursday extended exemptions to let an internationally backed nuclear deal with Iran go forward, even as it announced new sanctions to step up pressure.

President Donald Trump in 2018 pulled out of the accord negotiated by his predecessor Barack Obama, sending tensions soaring with Iran and leading Tehran to curb compliance, but European powers still back the deal.

Brian Hook, the US pointman on Iran, said that the United States would for another 60 days issue exemptions in its sweeping sanctions to let Russian and other companies implement it without fear of punishment by Washington.

The extension will "permit the continuation of nonproliferation projects that constrain Iran's nuclear activities," Hook told reporters.

"We will closely monitor all developments in Iran's nuclear program, and Secretary (Mike) Pompeo can end these projects as developments warrant," he said.

The exemptions affect the Bushehr nuclear power plant and the Arak heavy water reactor, with the nations in the accord working to ensure they are not put to military use.

The Trump administration has repeatedly extended the waivers but stopped doing so in November for another site, Fordo, in retaliation for Iran's lifting its level of uranium enrichment.

Iran's step, which still leaves its uranium well below weapons-grade, aimed to pressure Europeans to show tangible benefits of the nuclear deal despite the US imposition of sanctions.

A handout picture provided by Iran's Atomic Energy Organization in December 2019 shows the nuclear water reactor of Arak, south of capital Tehran
A handout picture provided by Iran's Atomic Energy Organization in December 2019 shows the nuclear water reactor of Arak, south of capital Tehran Atomic Energy Organization of Iran / HO

Just as it extended waivers, the United States announced new sanctions on the head of Iran Atomic Energy Organization, Ali Akbar Salehi.

The nuclear chief "personally inaugurated the installation of new, advanced centrifuges to expand its uranium enrichment capacity," Hook said.

The new sanctions freeze any US assets Salehi may have -- a move with little practical effect as Iran is already under severe sanctions.

Hook also announced the first transaction with Iran under a new financial channel aimed at easing humanitarian trade, with the sale to the country of cancer and transplant drugs.

Since Trump imposed sanctions on Iran, the United States has insisted that humanitarian goods are exempt. But with the United States vowing to end virtually all trade with Iran, many companies have been hesitant, fearing they could get caught up in US sanctions.

The United States announced the humanitarian channel in a joint initiative with Switzerland last year.

Critics voiced skepticism over the move as the United States was seeking exhaustive paperwork for each transaction and warning that it could use the documentation to impose sanctions if it finds wrongdoing.

Trump, a close ally of Iran's rivals Saudi Arabia and Israel, has vowed to fight Iran's regional role and earlier this month ordered the killing of Iran's most powerful general.