Former U.S. Secretary of State Hillary Clinton expressed serious concern while in office in 2010 after learning that North Korea showed an American nuclear scientist a uranium enrichment plant in operation, according to newly released Clinton emails.

Clinton called the development "very disturbing," according to a Nov. 13, 2010, email sent to her deputy chief of staff after he forwarded to her a report that Stanford University professor Siegfried Hecker wrote about his trip to the North that included a visit to the North's Yongbyon nuclear complex.

Hecker said in an email report to Ambassador Sung Kim, special envoy to the six-party talks on the North's nuclear program, as well as to then-special representative for North Korea policy Stephen Bosworth that the visit was a "shocker."

"They took us to Yongbyon and showed us a functioning 2,000 centrifuge uranium enrichment plant in operation and a small (light water reactor) under construction. The technical implications are significant and you understand the political implications better than I do," Hecker said.

Hecker's trip marked the first time that the North had publicly acknowledged the existence of the program and a facility for it, even though the communist nation had long been suspected of seeking a uranium-based nuclear weapons program.

The North has since bolstered its nuclear capabilities and conducted its fourth nuclear test last month, claiming that it successfully conducted its first hydrogen bomb test. Some experts have warned that the communist nation's nuclear arsenal could expand to as many as 100 bombs by 2020. (Yonhap)