BERLIN — The UN’s nuclear watchdog says Iran has breached another limit in its 2015 nuclear deal with world powers by stockpiling more heavy water than the accord allowed.

The International Atomic Energy Agency said Tuesday that Iran informed it on Nov. 16 that it had surpassed 143.3 tons allowed by the Joint Comprehensive Plan of Action. The agency confirmed Nov. 17 that Iran’s stockpile had reached 131.5 tons.

Heavy water helps cool reactors, producing plutonium as a byproduct that can be used in nuclear weapons. Iran insists that its nuclear program is for peaceful purposes.

Tehran has been slowly stepping up violations of the nuclear deal to pressure world powers to provide more incentives to make up for the US withdrawal from the deal last year and renewed American economic sanctions.