A new U.N. Security Council report says that North Korea may have secret nuclear facilities other than the one already revealed by the communist state in Yongbyon.

The assessment was made based on conversations with Stanford University Professor Siegfried Hecker as well as its own investigations and analysis into the inner workings of the North’s nuclear program.

Hecker, the former director of Los Alamos National Laboratory, said he saw hundreds of centrifuges used to enrich uranium during a visit to North Korea last year.

Although the report disclosed no details of other secret nuclear facilities, it concluded that North Korea’s uranium enrichment work started back in the 1990s and that the centrifuges most likely were produced at a location other than Yongbyon.

The report also made ten recommendations to prompt North Korea to fully abide by U.N. sanctions imposed after its nuclear tests in 2006 and 2009.

The recommendations include the blacklisting of North Korean companies suspected of engaging in nuclear activities and imposing a travel ban on North Korean officials who are in charge of nuclear development.