U.S. President Barack Obama, who will pay his respects Friday at the infamous Buchenwald Nazi concentration camp in Germany, has been sent an open letter from a Japanese citizens' group calling for the international community to denounce North Korea's notorious gulag system and not just focus on Pyongyang's nuclear threat.

No Fence, a Tokyo-based association seeking the release of political prisoners in North Korea, where as many as 300,000 people are believed to be subjected to torture, hard labor and execution, said in the letter dated Monday that if the world does not recognize the horror taking place in the dictatorship, "we will be questioned by future generations on why we failed to apply the lesson of past crimes against humanity."

The letter was endorsed by representatives of various international human rights organizations, including U.S.-based Human Rights Watch, South Korea's Committee for the Democratization of North Korea and Japan's Association of the Families of Victims Kidnapped by North Korea.