TEHRAN — Iran announced on Tuesday that it had completed a new centrifuge assembly center at the Natanz nuclear site, in a first step to increasing its enrichment capacity.

While Iran said it would keep enrichment within limits set by the 2015 nuclear accord, the center’s opening seemed to signal that it could swing to industrial-level enrichment if that agreement, which the United States withdrew from last month, should further unravel.

The head of the Atomic Energy Organization of Iran, Ali Akbar Salehi, told state television that the center’s construction had been “in line with our safeguard commitments but not publicly announced.”

A spokesman for the Iranian nuclear agency, Behrouz Kamalvandi, said a letter had been sent to the International Atomic Energy Agency explaining the action. He also told the semiofficial Iranian Students’ News Agency that Tehran would increase its capacity to produce uranium hexafluoride, a feedstock for centrifuges.