(CNN) The Trump administration is extending waivers that allow countries signed up to the Iran nuclear deal to participate in civil nuclear projects with Tehran, but it's tightening the terms in an effort to increase pressure on the Iranian regime.

The US will extend waivers that allow the remaining parties to the 2015 deal to conduct research and do non-proliferation work at three sites in Iran without fear of facing sanctions, according to a fact sheet released by the State Department on Friday.

However, the waivers will be reduced from 180 days to 90, and State Department spokesperson Morgan Ortagus said in a statement that "assistance to expand Iran's Bushehr Nuclear Power Plant beyond the existing reactor unit could be sanctionable." Bushehr is one of the three facilities exempted from sanctions along with Arak and Fordow.

The US will also revoke two waivers that allowed Iran to ship its excess heavy water for storage and swap its enriched uranium for natural uranium, according to Ortagus.

"The decisions today enhance our ability to constrain Iran's nuclear program while pursuing maximum economic pressure," Brian Hook, the State Department special representative on Iran, told CNN in an interview Friday.

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