Former Cuban leader Fidel Castro, wearing his green military cap and clothing like the comandante of old, made his first speech before the Cuban public since falling ill in 2006 on Friday, warning of the threat of nuclear war.

He spoke from the same steps of the University of Havana where 60 years ago he stirred fellow students to political action in the beginnings of the revolution that eventually put him in power in 1959.

About 10,000 people, mostly students, filled the steps and nearby streets to listen to the man who led Cuba for 49 years before the illness that forced him to resign the presidency and, Castro said in a recent newspaper interview, nearly killed him.

They shouted Fidel, Fidel Fidel and applauded at several points during his nationally-televised address.

His speech was the latest in a string of appearances since the 84-year-old Castro re-emerged in July from four years of seclusion.

His 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, President Raul Castro.

As he has all summer, Castro warned that nuclear war is inevitable if the United States, in alliance with Israel, tries to enforce international sanctions against Iran over its nuclear program.

He slammed the United States, his long-time foe, for creating a system that threatens the survival of humanity.

The problem of people today, the more than 7 billion human beings, is to prevent such a tragedy from happening, he said.

In this, as in many struggles in the past, it is possible to be victorious, he said.

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.

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

He wore plain military clothing, without military insignia or stars.

Fidel 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.

The president did not attend his brother's speech.

Raul Castro, 79, has lately stepped up the pace of small economic reforms he has initiated to revive Cuba's struggling economy and, in his words, assure the survival of the revolution after its current generation of leaders is gone.

Some details of Fidel 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.

(Editing by Frances Kerry)