Thousands flocked to the University of Havana to see Castro deliver his first major public speech in four years [EPA]

Fidel Castro has warned against the dangers of nuclear war in his first public speech since undergoing surgery in 2006. 

Castro, 84, said a nuclear conflict was inevitable if the United States, in alliance with Israel, tried to enforce international sanctions against Iran over its nuclear programme.

Addressing around 10,000 people, mostly students, at the University of Havana on Friday, he slammed Washington for creating a "system that threatens the survival of humanity".

"The problem of people today, the more than seven billion human beings, is to prevent such a tragedy from happening," he said.

"Faced with the skeptics, our duty is to keep up the fight, I am convinced that a good number of people are becoming conscious of the reality."

The crowd shouted "Fidel, Fidel Fidel" and applauded at several points during the nationally televised address.

"In this, like in many battles of the past, we can win," he said in reference to his efforts to warn the world of the nuclear danger he fears.

Standing behind a podium at the top of steps, he spoke for about 40 minutes, far shorter than the hours-long speeches he once gave.

As has been his custom since resurfacing, he did not talk about Cuba's domestic issues.

Shifting roles

His speech was the latest in a string of appearances since he returned to the public eye in July, giving his first television interview in at least three years.

Castro's renewed public presence has raised questions about whether he could resume a larger role in running the country now officially led by his younger brother Raul Castro.

Last month, he spoke to a session of the National Assembly, again mentioning the possibility of nuclear war.

Castro ceded power to his brother after undergoing surgery for an undisclosed intestinal ailment in July 2006, then officially resigned as president in February 2008.

He remains head of the ruling Communist Party and has continued to play a significant behind-the-scenes role, but there has been no sign that his brother is not in charge of the government.

Raul Castro, the Cuban president, did not attend his brother's speech.

The Cuban president has lately stepped up the pace of small economic reforms he has initiated to revive Cuba's struggling economy and, in own words, assure the survival of the revolution after its current generation of leaders were gone.

Some details of Castro's illness have trickled out over the years.

Castro himself recounted how ill he was in an interview with a Mexican newspaper last month, describing being close to death.