American negotiators insisted on “limiting” Iran’s enrichment of uranium and maintaining long-term verification of its nuclear program during indirect talks in Geneva on Thursday, a source familiar with the matter told CNN.

Iran presented “detailed proposals” aimed at addressing U.S. concerns about its nuclear activities as well as “ideas” responding to Washington’s demands, the source said.

The nuclear talks have centered on the nuclear enrichment and trying to determine whether proposals offered by the Iranians would satisfy President Donald Trump’s demand that the country never obtain a nuclear weapon, according to people familiar with the discussions.

The U.S., led by Steve Witkoff and Jared Kushner, entered the talks with some demands: that Iran dismantle its primary nuclear facilities and that any agreement to halt nuclear enrichment last in perpetuity.

The Iranians, meanwhile, were expected to offer a proposal that would suspend nuclear enrichment for a period of three to five years before joining a regional consortium for low-level enrichment, one of the people said. As part of a deal, Iran would allow international inspectors to monitor its compliance.

Foreign Minister Abbas Araghchi has said Iran remained “crystal clear” that it would “under no circumstances ever develop a nuclear weapon”, while also recognizing the right of Iran’s people to the benefits of “peaceful nuclear technology”.