VIENNA—Iran carried out its threat to enrich uranium at its underground Fordow nuclear site, the United Nations atomic agency said Monday, confirming Tehran’s most serious step yet away from the 2015 nuclear deal.

Iran has taken a series of steps since July to step away from the nuclear deal, a set of calibrated responses to harsh U.S. sanctions designed to pressure Europe into helping Tehran economically and persuade the White House that its maximum-pressure sanctions policy is leading Iran back to the brink of being a breakout nuclear power, something President Trump has said he won’t accept.

Iran’s steps to ramp up its nuclear activities in breach of the nuclear deal have been designed to keep the international community divided, deliberately avoiding big actions that would have prompted Europe to withdraw from the deal and trigger the reimposition of sanctions.

Instead, its moves have been diverse and incremental, often timed to gain maximum international attention and sometimes to test the reaction of international powers overseeing Iran’s activities,

The resumption of uranium enrichment at Fordow, which Iran announced last week, was the most dramatic step. While the International Atomic Energy Agency said that Iran is, for now, only feeding a few hundred basic centrifuges with the material, the site is a heavily fortified, underground location that is considered impregnable by most conventional weapons.