LONDON — Iran has breached a crucial limit on the level of uranium enrichment set out in the 2015 nuclear deal, the country’s atomic energy agency said on Monday, as China, another signatory to the deal, accused the United States of “bullying” Tehran with crippling economic sanctions.

Behrouz Kamalvandi, a spokesman for Iran’s atomic energy agency, told the Iranian state broadcaster IRIB that the country had surpassed a limit of 3.67 percent uranium enrichment, and was prepared to go further.

Mr. Kamalvandi later told another Iranian news outlet, ISNA, that the enrichment level was above 4.5 percent.

Fredrik Dahl, a spokesman for the International Atomic Energy Agency, the organization tasked with monitoring Iran’s compliance with the limits set out in the deal, confirmed that Iran is enriching uranium above 3.67 percent, but did not specify to what level.