South Korea has expressed serious concern and regret over North Korea's declaration to begin uranium enrichment.

In a statement issued Saturday, the North said it will proceed with uranium enrichment for nuclear development and "weaponize" all the plutonium it produces.

Seoul blasted Pyongyang's provocative measures as a challenge to the international community's strong will to create a nuclear-free Korean Peninsula, and bring peace and stability to Northeast Asia.

South Korean Foreign Ministry spokesman Moon Tae-young also called the North's declaration intolerable.

Moon said that by declaring the start of the uranium enrichment process, the North has unwittingly admitted to the continued operation of a secret uranium enrichment program as suspected.