Italy's Red Brigades terrorists released another letter attributed to their kidnap victim former premier Aldo Moro, yesterday saying he has been "placed in an unsustainable situation" by his own Christian Democratic Party and calling for a prisoner exchange before it is too late.

The letter, addressed to Premier Giulio Andreotti and seven other Christian Democratic leaders, was released to newspapers in three Italian cities only minutes after Andreotti concluded a hard-line speech to the parliament ruling out negotiations with the left-wing kidnappers.

"We refuse in the strongest possible terms to accept any terms of blackmail," he said. "We consider it out irrefutable duty to apply the law, to search out those responsible for crimes and to punish them."

Christian Democratic leaders declared in a statement that Moro had written the letter under coercion and that "it is not morally ascribable to him."

Fellow politicians and family members attributed a first letter last week to Moro and there seemed little doubt that he signed the latest one.

In it, Moro comes close to begging his fellow party members, as well as the powerful Communists who support them in the parliament, to take rapid action.

Moro said he felt "a bit abandoned by all of you." He emphasized that because his family needs him he finds it hard to "pay for the entire Christian Democratic Party" and suggested that "some concessions would be not only fair but politically useful."

Moro's reference to a prisoner exchange, already mentioned in the letter to Interior Minister Francesco Cossiga last week, was echoed by his Red Brigades captors in their "communique, Number 4" that accompanied yesterday's letter.

While still refraining from making specific demands, the Marxist terrorists reminded their readers that "we have often affirmed that one of the fundamental points of our organization's program is that of the liberation of all communist prisoners and the destruction of the regime's concentration camps."

At present, 15 alledge members of the Red Brigades are on trial for subversion in Turin.Another 155 suspected members are also in prison, along with more than a hundred persons suspected of links with other terrorist groups.

Moro explained his appeal to the Communists by reminding them that he was the chief architect of the current goverment, one in which a Christian Democratic Cabinet receives formal support from the Communists for the first time in 30 years. He made references to "political utility" that were interpreted to reflect veiled threats by the terrorists that Moro's party could suffer if he were to divulge sensational political secrets.

The letter thus seemed to be another major step in the Red Brigades apparent attempt: to discredit the man who had been slated for the presidency of Italy when the post becomes vacant in December.

Yesterday's debate in parliament marked the first official public discussion of the kidnapping since Moro was taken on March 16 in a bloody ambush in which his five bodyguards died.

Addressing the Chamber of Deputies, Andreotti accused the Red Brigade of "bloody, barbarous attacks" against the Italian democratic system. He said there was no hard evidence indicating a foreign involvement, as suspected by many Italians. But he said, "There are one, or more, groups of people who worked in the darked, now death and destruction, and strike coldly according to their timetable of terror."