A photo taken on November 10, 2019, shows the Iranian flag at the Bushehr nuclear power plant in Iran, during an official ceremony to begin work on a second reactor at the facility. Credit: ATTA KENARE/AFP via Getty Images

(CNN)-- An informed Israeli source told CNN on Tuesday that there is a belief in Israel that Iran moved advanced centrifuges and part of its stockpile of high-enriched uranium to an underground fortified facility south of Tehran, after the twelve-day war last June.

Israeli intelligence had been monitoring the site, known as Mount Axe, for months or longer, because it was one of Iran's most important nuclear facilities.

The location of the fortified facility, deep underground, made it a difficult target for the United States and Israel, and its location in the center of the country made any ground operation extremely difficult.

CNN previously reported that Iran significantly intensified its efforts to isolate its stockpile of semi-enriched uranium, deliberately demolishing tunnels and laying mines at the entrances.

Iran possesses nearly 440 kilograms of high-enriched uranium, and disposing of this uranium remains one of the main war objectives for both Israel and the United States.

Israel assesses that Tehran moved part of the uranium to Mount Axe.

The Wall Street Journal was the first to report the Israeli assessment.

An Israeli source said construction and activity at the site increased after the war in June 2025. Shortly before the end of that war, the United States conducted strikes on three Iranian nuclear sites – Fordow, Natanz, and Isfahan – but did not hit Mount Axe.

In December, a senior Israeli official told CNN: "There are ongoing signals, statements, and messages indicating that Iran is rearming, strengthening its capabilities, and preparing to launch an attack."