Washington (CNN) EU Foreign Policy Chief Federica Mogherini said the Iran nuclear deal is not open for renegotiation and that US lawmakers have indicated that their goal, as they amend legislation related to the pact, is to keep the US in compliance with the multilateral agreement.

"Renegotiation is not an option," Mogherini told reporters in Washington, citing the 12 years of talks between multiple parties on deeply complicated issues. She added a warning that the US would be isolated if it pursues this course.

"I have to tell this very clearly," Mogherini said. "For Europeans as well as for the others that are sitting in the Joint Commission... meaning, the E3, Russia, China, renegotiating part of the agreement is not an option."

The EU's top diplomat also pushed back against a common criticism from US opponents of the deal who argue that so-called "sunset" provisions mean that Iran will eventually be able to resume its work toward a nuclear weapon.

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"The sun doesn't set when it comes to nuclear commitments," Mogherini said. "Article three ... says Iran will never develop a nuclear weapon."

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