Saying he spoke as a friend, Castro wrote in Cuban state media that North Korea, led by 30-year-old Kim Jong-un, had shown the world its technical prowess and now it was time to remember its duties to others.
North Korea, which along with Cuba is one of the world's last communist countries, has been ratcheting up pressure by declaring war on neighbor South Korea and threatening to stage a nuclear strike on the United States.
Few observers believe it will actually attack anyone, but Castro has become an anti-nuclear advocate in recent years.
"The Democratic People's Republic of Korea was always friendly with Cuba, as Cuba always has been and will continue to be with her," Castro wrote, using an almost paternalistic tone.
"Now that it has demonstrated its technical and scientific advances, we remind it of its duties to other countries who have been great friends and that it would not be just to forget that such a war would affect in a special way more than 70 percent of the world's population," said the 86-year-old, who turned Cuba communist after taking power in a 1959 revolution.
Castro called the present situation on the Korean Peninsula "incredible and absurd," but said "it has to do with one of the gravest risks of nuclear war since the Crisis of October (Cuban Missile Crisis), 50 years ago."
He led Cuba through the October 1962 showdown when the United States and Soviet Union nearly went to war over the placement of Soviet nuclear missiles on Cuba, 90 miles (144 km) south of Florida.
At one point, Castro wrote a letter to Soviet leader Nikita Khruschev urging a nuclear attack on the United States, which he assumed was about to invade the Caribbean island.
Cooler heads prevailed as Khruschev and President John F. Kennedy reached an agreement in which the Soviet missiles were removed and the United States promised never to invade Cuba.
Castro ruled Cuba for 49 years before age and ill health forced him to step down in 2008.
He was succeeded as president by younger brother Raul Castro, 81, but remains a power behind the scenes and writes occasional columns for Cuban press.
The elder Castro also said the United States had the responsibility to prevent war, which he said if unleashed would make President Barack Obama look like "the most sinister person in the history of the United States."