Some of Iran's most highly enriched uranium, close to weapons grade, was stored in an underground area of its nuclear site in Isfahan, the U.N. nuclear watchdog said in a confidential report sent to member states on Friday and seen by Reuters.

It is the first time the International Atomic Energy Agency has reported where uranium enriched to up to 60% purity, close to the 90% of weapons grade, has been stored.

The tunnel complex's entrance was hit in U.S. and Israeli military strikes in June, but the facility seems largely unharmed, diplomats say.