Hours after sanctions went into effect that ban most trade with Iran, the State Department said it was exempting projects set up through the 2015 nuclear deal negotiated under former president Barack Obama.

The international activities at Bushehr, Iran's only nuclear power station, as well as the Fordow enrichment plant and the Arak heavy water reactor will be allowed "to continue under the strictest scrutiny to ensure transparency and maintain constraints on Iran," the State Department said in a statement.

"This oversight enhances our ability to constrain Iran's program and keep pressure on the regime while we pursue a new, stronger deal," it said.

The State Department said the waivers were "temporary," without specifying a timeframe, and "conditional on the cooperation of the various stakeholders."

The 2015 agreement promised that world powers would assist Iran in developing civilian nuclear energy -- the clerical regime's stated goal for its atomic program.

Russia has supplied fuel for the Bushehr reactor. The Arak site, which could eventually be used to produce plutonium, is being redesigned under the deal to ensure it does not, with spent fuel to be shipped out.

Russia is also working with Iran on isotope production at Fordow to ensure that the site works toward medical purposes rather than uranium enrichment.

The other parties to the deal -- Britain, China, France, Germany and Russia along with the European Union -- say that the accord remains in force and is working, noting that UN inspectors report that Iran has complied.

President Donald Trump has called the agreement a "disaster" and, as of Monday, the United States will sanction countries and companies that do business with Iran or buy its oil.

Secretary of State Mike Pompeo said Monday the United States wants Iran to undertake a "180-degree" change that includes cutting off support for regional proxies such as Hezbollah and ending missile tests.