The Soviet Union intensified its condemnation of the Israeli invasion of Lebanon tonight, with Tass warning Israeli leaders that their actions "undermine the foundations of the very existence of Israel as a state."

The government news agency also stepped up its attack on the Reagan administration, accusing it of employing "the most adventurist and fascist methods" to protect "the interests of U.S. monopolies" in the Middle East.

Tass said the Israelis intend to "exterminate the Palestinian people and thereby 'resolve' the Palestinian problem."

The dispatch, datelined Damascus, said that "criminal deeds" of "the Zionists" are imperiling the country. "Their current venture is sure to turn into a catastrophe for themselves," Tass said.

Another Tass commentary compared Israeli actions in Lebanon to "the atrocities of Nazi barbarians" and said that Israel "has obviously set out to physically annihilate the Arab population of Palestine" in collusion with the United States.

Another dispatch said Israeli Prime Minister Menachem Begin and his defense minister, Ariel Sharon, "are leading their people into a deep political impasse and they are in for a stern punishment for their heinous crimes."

It continued: "But they are obviously under delusion, resorting to the criminal method of genocide. They will not attain their aim. The Palestinian resistance movement, as part of the national liberation movement, exists and will continue to exist not only in Lebanon but also in the other countries of the Arab world."

The commentaries, which seem to reflect the government's rage and frustration over Israel's stated intention to destroy the Palestine Liberation Organization in West Beirut, suggest that Soviet humiliation over destruction of Moscow's PLO clients could lead to a reassessment of Soviet policy toward Israel.

Until now, the Soviet Union has not questioned Israel's existence as a state. On the contrary, Moscow has consistently supported the existence of Israel within its pre-1967 borders.

Whether tonight's Tass attack was hyperbole could not be determined. But the Soviets, clearly reluctant to get involved in Lebanon, seem to have mounted a political drive to try to rescue their Palestinian allies.

Despite the harsher tone of Soviet pronouncements in the past 24 hours, there have been no suggestions that Moscow plans any concrete action to help the encircled Palestinian guerrillas in West Beirut.