Castro, who stepped down as Cuban president in 2008, said in an article in the official Communist Party newspaper Granma the United States has a duty to ensure there is no conflict, the British broadcaster ITV reported.
He said the current state of tension on the Korean peninsula creates the greatest danger of nuclear war since the Cuban Missile Crisis in 1962.
"If a war breaks out there, there would be a terrible slaughter of people with no benefit for either of them," Castro said.
A nuclear war would affect 70 percent of those living in the world, Castro said.
Castro called on North Korean leaders to remember "their duties with those countries that have been their great friends."
Castro, 86, led the revolution that put a communist government in place in Cuba in 1959. His regime has survived the breakup of the Soviet Union and China's turn to state capitalism as well as five decades of U.S. embargo.