Recovering Iran’s remaining highly enriched uranium stockpile believed to be sitting in a storage facility deep underground, an objective the Trump administration has been discussing, would require a significant number of US ground troops beyond a small special operations footprint, seven current and former officials familiar with the military planning told CNN.

The US bombing campaign that struck three Iranian nuclear facilities last June did not destroy all of that country’s highly enriched uranium stockpile, the material needed to create a nuclear weapon. Much of it is believed to be at Iran’s Isfahan nuclear site, said three of the sources. President Donald Trump has made complete elimination of Iran’s nuclear capability one of his stated aims of the war.

If the Trump administration went ahead with an operation to retrieve the uranium, it could mark the first major commitment of US ground forces as part of the campaign, an escalation that would put a large number of troops in harm’s way as part of a complex mission to move or render safe tons of highly radioactive material.

The uranium at Isfahan is accessible to the Iranians, who had been working for months after the US military strikes last year to clear the rubble of the facility’s aboveground structures and access the underground tunnels where the uranium was hidden, two of the sources said. UN nuclear watchdog chief Rafael Grossi said on Monday that around 200 kilograms of Iran’s highly enriched uranium is probably still at Isfahan, with some also at Iran’s Natanz nuclear facility.