Israel warned yesterday that it would not stand by idly while Lebanese Christians are killed.

The Israelis, who invaded southern Lebanon this past spring in response to a terrorist attack, said they are considering what steps to take to stop the bloodshed in Beirut, Lebanon's wartorn capital.

Included in the warning was a caution that a change in the tenuous poticial-military balance in Lebanne could spill over and affect the entire framework of Middle East diplomacy.

As if to reinforce the warning. Israeli Air Force jet fighters screamed over Beirut at rooftop level in a "reconnaissance" overflight that Israeli military officials said also was intended to boost the morale of the besieged Christians.

As Syrian artillery shells pounded the Christian quarter of Beirut, three top-ranking military and government leaders issued blunt warnings to Syria that Israel considers the Beirut attacks a threat to the security of southern Lebanon and the Israeli border.

Eliahu Ben-Elissar, director general of Prime Minister Menachem Begin's office, accused Syrian forces of "performing a massacre" on the Christian population in Beirut, and said Israel views the events there with "concern and gravity."

"We have promised and committed ourselves not to let the Christian population be annihilated in Lebanon. This is a lesson we all should learn about what can happen when a small and weak country is in a struggle for its independence - actually today for its life," Ben-Elissar said.

He added, "We shall consider steps to be taken in order not to let the Christian population be annihilated."

Israeli Defense Minister Ezer Weizman issued a similar warning, saying that seizure of Lebanon by Syrian forces would force an Israeli "reassessment" of the situation there.

Weizman, in a radio comment, said that new answers are now required from those who offer advice to Israel with respect to the Middle East peace negotiations.

And Israeli Maj. Gen. Shlomo Gazti, chief of Israeli Army intelligence, said that a Syrian success in Beirut and deployment of Syrian troops along the border would have "serious implications" for Israel. He said that Lebanon would then have to be regarded as a confrontation Arab state.

Moreover, he said, a Syrian success might change the balance of forces in the inter-Arab scene generally, and between Syria and Egypt in particular. This development, Gazli said, would require Israel's Cabinet to decide whether the Syrians had crossed the "red line."

Israelis have often spoken of a "red line" serving as a trigger for Israeli involvement in Lebanon. In the past, this "line" has generally been interpreted as the Litani River in southern Lebanon.

Speaking to reporters at a briefing in Tel Aviv, Gazi said 40,000 Syrian peacekeeping forces were in Lebanon. He said half of them were in Beirut preparing for their attack on the Christian quarter. They have been reinforced with a variety of artillery pieces and mortars, he said.

Gazit's estimate of the number of Syrians surrounding Beirut's Christian quarter was higher than officials had previously estimated. Gazit said that at least 400 are dead in the Christian quarter, and that many more wounded cannot be evacuated or receive medical treatment, the Israeli figure of 400 dead is more than double the fatality figure generally accepted in Beirut.

"In a most systematic and cruel way, they are destroying house after house, target after target." Gazit said, adding that "the main purpose of the attack is to destroy any military capability on the Christian side" and to wipe out any resistance to Syrian control of Beirut.

Asked why the seven Israeli Kfir jet fighters overflew beirut, rattling and breaking windows but causing no other damage. Gazit said, "We do this from time to time. We just wanted to know better and closer" what was happening on the ground. That the over flight also "boosted a little bit the low moral" of the Christians.

When asked whether militarily, Gazit said, "The (Israeli) government will probably discuss the possibility and make a decision." He said that Lebanon is "very close by" Israel, and that whatever happens there is of "great significance" to Israel.

"This will have and does have reprecussions in south Lebanon and we must be concerned about that," Gazit said. When asked whether the Beirut siege would affect the Middle East peace negotiations, Gazit said, "It will probably change our attitude at least as far as security arrangements are concerned."

Gazit said he did not know of any direct link between the artillery attacks and Israel's withdrawal from southern Lebanon on June 13 following its invasion a month earlier.

He said the Christians in the south were watching the Beirut developments and are worried about their own fate.

Israel's close attachment to Christian Lebanese forces has been apparent before, most recently just before the June 13 withdrawal when Israeli troops turned key positions over to local rightist Christian militas to the chargin of the commander of the U.N. peacekeeping force.

That step led to suggestions that Israel intended to circumvent the United Nations and retain control over the border area by proxy, since it is known to supply the Phalangist forces there with arms and money.

It is too early to tell how far Israel will go to protect the Christians in Beirut, but the statements of officials here suggested that the Israeli government views of the Syrian attacks as not only a potential threat to the security of southern Lebanon, but as a symbol of oppression of a minority.

Gazit, for example, referred to "the implications for Israel that one can crush a minority in the Middle East without any of one doing anything about it."