"If war breaks out there, the peoples of both parts of the peninsula will be terribly sacrificed, without benefit to all or either of them," he said in a column published in Cuban state media.
"Now that (North Korea) has demonstrated its technical and scientific achievements, we remind her of her duties to the countries which have been her great friends, and it would be unjust to forget that such a war would particularly affect more than 70 per cent of the population of the planet."
Castro, 86, reminded the US of its duty to avoid a clash, amid mounting tensions this year between North and South Korea.
"If a conflict of that nature should break out there, the government of Barack Obama in his second mandate would be buried in a deluge of images which would present him as the most sinister character in the history of the United States," he said.
"The duty of avoiding war is also his and that of the people of the United States."
Cuba is one of the last remaining allies of the communist government in Pyongyang.
"The Democratic People's Republic of Korea was always friendly with Cuba, as Cuba has always been and will continue to be with her," Castro wrote.
"I had the honour of meeting Kim Il-sung, a historic figure, notably courageous and revolutionary."
Kim Il-sung was the founder of North Korea and grandfather of Kim Jong Un, the new leader of the reclusive Pyongyang regime.
Tension ratcheted up this week on the peninsula, as North Korea has threatened nuclear strikes and moved missiles, with the South and the US positioning missile defences in response.
VIDEO: North Korea warns embassies over safety
