At the height of the Cuban missile crisis, Fidel Castro was convinced a U.S. invasion was imminent and urgently asked Soviet leader Nikita S. Khrushchev to launch an immediate nuclear strike against the United States, a key Soviet source reportedly told some American colleagues.

The Cuban leader was said to have sent his message to Khrushchev after spending the night of Oct. 26, 1962, in the bomb shelter of the Soviet Embassy in Havana, so convinced was he that war was about to break out.Castro supposedly told Khrushchev the invasion would come within 48 hours.

The account from the Soviet source was relayed by Pierre Salinger, who was President Kennedy's press secretary at the time of the 1962 crisis. It came from a weekend meeting in Moscow of U.S., Soviet and Cuban participants and scholars who have studied the world's brush with nuclear war 26 years ago.

Salinger would not identify the Soviet official, who was said to have imparted the information in a private conversation, and the account was later denied by at least two Soviets who took part in the conference.

Among the revelations for Americans attending the session was a Soviet general's disclosure that 20 missiles and warheads - which he said constituted one-third of his country's nuclear arsenal at the time - were on Cuba at the time. However, he and another Soviet official said the warheads had not been mounted on missile launchers and were not ready for firing.

An American source said, however, that he and other former U.S. officials learned for the first time at the conference that the warheads not only were on the island but could have been mounted and fired within hours. The source asked not to be identified.

The New York Times on Monday quoted unnamed Americans as saying the report about Castro requesting the nuclear strike originated with Sergei Khrushchev, son of the former Soviet premier. It said Sergei Khrushchev subsequently denied making the comment.

The report also was denied by Aleksandr I. Alekseyev, Soviet ambassador to Cuba in 1962.

Salinger, in a telephone interview with The Associated Press and on ABC News, for which he is a special correspondent, said the conference that brought together participants from all three sides for the first time showed the world was "closer to nuclear war than we believed at that time."

Other participants said the discussions also disclosed the Soviet nuclear arsenal was smaller than American intelligence believed at the time and that there were four times more Soviet troops in Cuba than the Kennedy administration thought.

A Cuban official disclosed that 270,000 Soviet and Cuban troops had been ready to go to war with the United States and that 100,000 casualties were expected, former U.S. Defense Secretary Robert McNamara said Sunday.

At a news conference wrapping up the conference Sunday, he said the figures on Cuba's war readiness and casualty estimate were provided by Jorge Risquet, a member of Cuba's ruling Politburo.