A power failure that appeared to have been caused by a deliberately planned explosion struck Iran’s Natanz uranium enrichment site on Sunday, in what Iranian officials called an act of sabotage that they suggested had been carried out by Israel.

The blackout injected new uncertainty into diplomatic efforts that began last week to salvage the 2015 nuclear deal repudiated by the Trump administration.

Iran did not say precisely what had caused the blackout at the heavily fortified site, which has been a target of previous sabotage, and Israel publicly declined to confirm or deny any responsibility. But American and Israeli intelligence officials said there had been an Israeli role.

Two intelligence officials briefed on the damage said it had been caused by a large explosion that completely destroyed the independent — and heavily protected — internal power system that supplies the underground centrifuges that enrich uranium.