The Italian government yesterday firmly rejected a new demand by Red Brigades kidnapers, despite an appeal by their hostage, former premier Also Moro, who warned, "We are at the moment of slaughter."

The letter from Moro, written with a shaky hand, said: "We are almost at zero hour. We are at seconds rather than minuites from it."

Moro accused the Christian Democratic Party, which he heads, of restoring capital punishment by allowing the kidnapers to kill him. He said he would forgive no one and asked that no party or government officials attend his funeral.

The latest terrorist ultimatum said Moro would be killed if the government failed to give an "immediate and affirmative" reply to their new demand - the release of 13 jailed leftists. It was the first specific demand since Moro was kidnaped March 16 in an attack that left his five police bodyguards dead.

Yesterday's message, distributed to news media in Genoa, Milan, Turin and Rome, was described by government sources as "probably authentic." It was the first word from the Red government allowed an earlier deadline to expire without complying with a demand for the release of unnamed "Communist prisoners."

The government broke a three week silence to issue a statement saying the latest demand was "unacceptable," and indicating that the Red Brigades have no intention of freeing the former premier.

A source close to one key Cabinet minister said last night that the government's hard-line position was an attempt to deny the Red Brigades fulfilment of their major goal, that of winning the type of moral or legal recognition that many view the Palestine Liberation Organization as having acquired in recent years.

He said he now expected the terrorists to carry out their threat to kill Msoro, perhaps as early as today, a national Italian holiday marking the uprisings that liberated many major northern Italian cities from the German army in 1945.

"If the terrorists kill Moro, it will be because they are weak and because they have lost this round of this terrible contest," he said.

"But for Italy," he added, "it is nevertheless a very sad moment indeed."

Yesterday's letter from the kidnappers, who describe themselves as "Marxist-Leninist Communists," provided a list of the prisoners they want released. Among them are Renato Curcio, 36, a founder of the Red Brigades, and three other members of the group who are on trial in Turin on charges of subversion.

The other nine prisoners are two other Red Brigades members, including Cristoforo Piancone, 28, who was wounded in a Red Brigades killing two weeks ago; three members of the Genoa based "October 22" group, one of the earliest of the recent crop of terrorist bands; three members of the now defunct, Naples based "Armed Proletarian Nuclei," and a well known bank robber, Santo Notarnicola, who became a terrorist sympathizer while in jail. Six of the persons on the list are serving sentences for murder or attempted murder.

Government sources said yesterday afternoon that there would be no softening of its hardline position, first expressed on April 4, ruling out a prisoner exchange. The Christian Democrats pledged Friday to uphold the "democratic state, it institutions and its laws," which do not allow prisoner exchanges.

Yesterday, Enrico Berlinguer, secretary of the powerful Communist Party that gives the government essential partiamentary support, said in Florence that his party wanted "a clearcut rejection of any yielding to the terrorists."

It has been also pointed out repeatedly in recent days that because of Italy's system of a separation of powers, the government does not have the power to release prisoners sentenced by the courts.

Four years ago when the Red Brigades kidnaped Genoa judge Mari Sossi and offered to exchange him for several jailed terrorists belonging to the "October 22" band, a lower court agreed to the exchange. Following Sossi's release, it was overreuled.

In yesterday's message, the Red Brigades asaid there would be no "analogous" solution for "the prisoner, Aldo Moro." His death sentence, it said, would be carried out immediately if the government failed to release the 12 men and one woman on the Brigades' list.

In their message, the terrorists also "took note" of this weekend's appeals for Moro's life from Pope Paul, and U.N. Secretary Genral Kurt Waldheim, but said that they could respond to them only if similar appeals we madefor the 12 prisoners on their list.