Soviet leader Nikita Khrushchev promised strong support to Premier Fidel Castro in the 1962 Cuban missile crisis but months later sent a long, rambling apology for backing out in the face of U.S. resistance, newly released documents show.

"Because of the situation that has arisen, we have instructed the Soviet military representatives stationed in Cuba on the need to take the necessary measures and to be at full readiness," Khrushchev wrote around Oct. 22, 1962.But three months later, after the Soviet Union had pulled its missiles from Cuba in response to ultimatums from the United States, Khrushchev felt the need to write to Castro to try to mend any hard feeling that may have developed from the withdrawal.

"I have already told you, comrade Fidel, that at this time there exists . . . a certain amount of resentment and that this harms the cause," Khrushchev wrote Castro Jan. 31, 1963.

Translations of the documents, released recently by the Cuban government, were handed out Tuesday at a news conference ending a five-year U.S.-Russian-Cuban project studying the missile crisis that was conducted by the Center for Foreign Policy Development at Brown University. Also released was a draft agreement to deploy Soviet nuclear weapons in Cuba.

On Tuesday former Defense Secretary Robert S. McNamara said the CIA badly underestimated the number of Soviet troops in Cuba and also failed to detect the presence of 24 medium-range missiles with nuclear warheads on the island during the 1962 crisis.

CIA spokesman Mark Mansfield had no comment on McNamara's allegations.

"We were told by the Russians that the Soviet forces in Cuba in October 1962 - numbering 42,000 men instead of the 10,000 reported by the CIA - possessed 36 nuclear warheads for the 24 intermediate-range missiles that were capable of striking the United States," McNamara said.

"At the time, the CIA had stated they did not believe there were any nuclear warheads on the island."