U.S. officials and independent experts said Iran appears to be building a tunnel complex inside a mountain, The Washington Post reported Monday.

Recent satellite photos indicate a major digging project near the nuclear facility in central Iran at Natanz, where Iran is enriching uranium in defiance of international protests, the Post reported.

Officials at the International Atomic Energy Agency, the Vienna-based U.N. watchdog that monitors Iran's nuclear program, confirmed Friday they have broached the subject with Iranian officials.

"We have been in contact with the Iranian authorities about this and we have received clarifications," IAEA spokeswoman Melissa Fleming told the Post.

A tunnel complex would narrow the options for a pre-emptive military strike to destroy Iran's nuclear program, U.S. military experts told the Post.