In an article for Granma, the communist party's newspaper, the 86-year-old wrote: "If a war breaks out there, there would be a terrible slaughter of people with no benefit for either of them."
Castro said the situation in Korea presented the most serious risk of nuclear war since the Cuban missile crisis in 1962 - where there was a two-week standoff between Russia and the United States over the placing of nuclear weapons on the Caribbean island.
He added that the North Korean government "has demonstrated its technical and scientific advances" but reminded "them of their duties with those countries that have been their great friends."
Castro said a war on the Korean peninsula would affect "more than 70 per cent of the planet's population" and added that the "duty" to avoid the conflict was in the hands of Washington.
Through her tears, a North Korean woman told me how she had to escape: risk death while running across the border or die of starvation.
Memories of the Korean war are never far away in South Korea, where residents have grown used to living with the constant threat of war.
North Korea said it will withdraw workers from the Kaesong factory park run jointly with South Korea and suspend all operations there.