Former Cuban President, Fidel Castro, has decided to speak out against North Korea in the wake of a possible nuclear war.

The ex-president urged North Korea to avoid atomic warfare at all costs, reports The Los Angeles Times.

This is "one of the most grave risks of nuclear war" since the Cuban Missile Crisis years ago, the former leader said.

Recently, North Korea stepped up its' talks against South Korea and their allies, the United States, declaring a state of war on both countries. The Kim Jong Un-led party has also said they have restarted a nuclear reactor they shut down five years ago in accordance with a U.S. deal.

Although analysts say North Korea is not yet capable of a nuclear attack, the former Cuban president was still worried.

"If war breaks out there, the people on both sides of the [Korean] peninsula will be terribly slaughtered, without any benefit for either of them," Castro wrote in the Cuban Communist Party newspaper Granma.

He also recalled the "honor of knowing" the deceased North Korean leader Kim Il Sung, who was "a historic, strikingly valiant and revolutionary figure." Il Sung is also the grandfather of Jong Un.

"It would not be just to forget that such a war would affect, in a special way, more than 70% of the world's population," Castro wrote.

The former communist dictator, who led his country through the Cuban missile crisis and later denounced nuclear war, also sent warning shots in the direction of President Barack Obama if the United States fails to stop such a war.

Obama would, "look (like) the most sinister person in the history of the United States," Castro wrote.