Prime Minister Menahem Begin proclaimed publicly today that Israel is providing training, supplies and artillery support to Christian militiamen in southern Lebanon, calling it Israel's "duty" to prevent what he called an attempt at "genocide" by Palestinian forces in Lebanon.

Begin blasted France for failing to intervene in behalf of Christian villagers and soldiers who have been under siege in southern Lebanon for months. He said that "Christian France, the patron of the Maronite Christians for ages, had sold them down the river" to curry favor with the Arab oil-producing countries who support the Palestinians.

It has been known for some time that the Israelis were providing ammunition and training as well as medical and economic assistance to Christian forces in Lebanon, across what is known as the "good fence."

This is apparnetly the first time that an Israel leader has acknowledged that Israeli troops are actually participating in the fighting or has attempted to turn what some observers see as an opportunistic policy into a national moral commitment.

The Palestinian news agency WAFA reported in Beirut that Israeli troops, backed by tands and artillery, crossed the border and tried to storm a village less than a mile from the frontier. The agency said Palestinian and lefist Lebanese forces repelled the attack, inflicting heavy casualties on the Israelis.

[There was no independent confirmation of the report. Israeli military authorities in Tel Aviv said they knew nothing of such an attack.]

Begin toured the Lebanese border area yesterday. This morning he told delegation of American and Canadian Israel Bond Drive leaders that the Christians in southern Lebanon "without our help would long ago have been wiped out totally."

In pledging continued support for the Christians, Begin neglected to mention the strong evidence that it was actually they who started the Lebanese civil war, attacking the Palestinians refugees they thought were trying to take over the country.

His remarks about Lebanon were part of a strongly worded denunciation of the Palestine Liberation Organization as a band of terrorists bent on "genocied" against both the Lebanese Christians and the Jews in Israel.

On the eve of the arrival of U.S. Secretary of State Cyrus Vance, Begin reaffirmed that htis country would never engage in negotiations with the PLO, and insisted that if the United States wanted such negotiations. Israel would invoke a "written agreement" that give it veto power over any change in the composition of the delegations to a Geneva peace conference.

There has been no public indication that the United States is asking the Israelis to admit the PLO to the Geneva meeting, but the intensity of Begin's prolonged declaration of refusal prompted speculation that something is in the wind.

"Why should the people of this little country negotiate with their destroyers?" he asked.

Began said that Israel's "right" to veto any Palestinian partcipation at Geneva was "there ro be used," and added: "This is the test of the commitment" made by the United States.

Describing the Christian villages of southern Lebanon as "islands in a Moslem sea," Begin said that the Israelis have engaged in artillery duels with Palestinians who have been bombarding the villages.

He said the water supply to the villages has been cut off by the shelling, and added that the Israelis would begin work today on a new water line across the border to restore water service.

Begin spoke scornfully of Frances and its pro-Arab policies, but he seemed to be addressing the United States as well when he said. "The Arabs can't drink the oil, they have to sell it."

He expressed the hope that "In God's good time the free world will set itself free of dependence on the mighty power called Abu Dhabi."