The nuclear plant was rebuilt at the site of an earlier facility dismantled in 2008 as part of an agreement with the international community.
The Obama administration said it would amount to an unacceptable United Nations violation.
It was unclear as to why North Korea provided Hecker with access to the secret facility.
Chairman of the Joint Chiefs of Staff Adm. Mike Mullen said Sunday that the revelation of North Korea's stepped up nuclear program is more evidence of the country's belligerent behavior.
"From my perspective, it's North Korea continuing on a path which is destabilizing for the region. It confirms or validates the concern we've had for years about their enriching uranium, which they've denied routinely," he said. "They are a country that routinely we are unable to believe that they would do what they say."
The Associated Press contributed to this report.