A top aide to Christian militia leader Bashir Gemayel said today that he has given up hopes for a peaceful settlement of the Beirut crisis and predicted that it would be very hard to forge a united Lebanon if Israel launches an all-out military assault on West Beirut.

Maintaining that negotiations to achieve a withdrawal of Palestinian guerrillas were at "position zero," Karim Pakradouni said that the Israelis might attack within several days. Gemayel's rightist Phalange militia is effectively allied with the Israelis, although it is not necessarily kept abreast of Jerusalem's military plans.

"This cease-fire is the silence before the storm," Pakradouni said in an interview. "Perhaps this is the last cease-fire."

Interviewed after the weekly meeting of the Phalangist Party's political bureau, of which he is a member, Pakradouni also disclosed that Christian leader Gemayel has made conciliatory and secret overtures during the past several days to Lebanese Moslem leaders in besieged West Beirut to form a unified Christian-Moslem government.

"The chance for reconciling the Lebanese factions is now," Pakradouni said. "It will just be too difficult after the Israelis have attacked West Beirut and attacked the Syrians in the Bekaa . . . . There will be too many bad feelings afterward."

The men approached, according to Pakradouni, included Walid Jumblatt, leader of a loose leftist coalition called the National Movement and of the Moslem Druze sect; Nabih Berri, head of the Moslem Shiite Amal movement and traditional Sunni Moslem leader Saeb Salaam, who six times has been prime minister. He did not say how they responded.

Pakradouni said the current negotiations on evacuating the Palestine Liberation Organization from Beirut have "gone back to position zero, and special American envoy Philip C. Habib no longer has a political solution." There have been signs previously that the Christian militia would like to see the talks fail because an Israeli attack would seriously hurt the militia leader's rivals, the PLO and leftist Lebanese Moslems.

"Habib no longer has much credibility with the Lebanese and Israelis because it has been" three weeks since the talks began on the withdrawal of the PLO, Pakradouni continued.

Initially, Pakradouni said, Habib thought it would be relatively easy to get the PLO to withdraw because they had been defeated. Through all the camouflage of myriad negotiating demands, PLO leader Yasser Arafat has named two basic positions, the Phalangist added.

"Arafat's positions are, 'I am not leaving' or 'I am not leaving before I receive direct recognition from the United States government,' " Pakradouni said. "Habib cannot deliver American government recognition so he will not be able to budge Arafat." Successive administrations have maintained that there will be no recognition of the PLO until the PLO accepts Israel's right to exist.