S enior Israeli officials are acknowledging that some enriched uranium may have survived the powerful U.S. strikes on Iran's key nuclear sites last month.

The process of uranium enrichment increases the concentration of U-235, an isotope that can sustain a nuclear chain reaction. Beginning in 2006, the U.N. Security Council called on Iran to suspend all uranium enrichment, but it endorsed the JCPOA on July 20, 2015.

The Pentagon on Wednesday sharpened its assessment of Iran's nuclear program, declaring that Iran's ability to build a nuclear weapon following the U.S. strike on its nuclear facilities is "closer to two years" away.

The appraisal that Iran’s nuclear hopes had been set back 1 to 2 years by U.S. and Israeli bombings was the latest in a shifting series of pronouncements.

Israeli Prime Minister Netanyahu meets with President Trump to discuss Iran strikes, Gaza conflict and Abraham Accords amid new era of U.S.-Israel strategic partnership

We’re thinking probably closer to two years — like degraded their program by two years,” Pentagon spokesperson Sean Parnell said Wednesday.

US airstrikes on Iran set back the regime’s nuclear program between one to two years, but likely “closer to two,” the Pentagon’s top spokesperson said Wednesday. “Our assessment of the battle damage around Fordow, Natanz and Isfahan remains unchanged,” Pentagon spokesman Sean Parnell told reporters during a press briefing.

A scholar of intelligence and strategy explains why battle damage assessments are so challenging – and why the process has become politicized.

The intelligence community has not finalized its battle damage assessment for the effects of the U.S. military's strikes on three of Iran's nuclear facilities.

President Donald Trump said on Friday that Iran had not agreed to inspections of its nuclear program or to give up enriching uranium.