Amid increasing nervousness in Beirut and in Washington about the possibility of an Israeli attack on Palestinian positions in southern Lebanon, opposition secretary Haim Bar-Lev, a former Army chief of staff, said today that Israel should attack only if the quiet along Israel's northern border is broken, or if the Syrian Army moves into the border salient controlled by Lebanese Christian militias.

In an interview on Israeli state radio, Bar-Lev said the current situation in Lebanon does not warrant a full-scale Israeli attack against the Palestine Liberation Organization.

Bar-Lev's remarks, and a similar appeal for caution by another former chief of staff, Mordechai Gur, prompted a reaction by Prime Minister Menachem Begin, who called the statements "acts of irresponsibility of the highest degree."

Speaking for the prime minister, Begin's spokesman, Uri Porat, said tonight: "They Bar-Lev and Gur are talking of facts of which they do not know, and this is a low point which the opposition has reached."

Both Bar-Lev and Gur are members of the Israeli parliament's influential Foreign Affairs and Defense Committee.

Bar-Lev's comments attracted widespread attention here tonight because he was a participant in an unusual meeting Tuesday between Begin and leaders of his Likud coalition and leaders of the opposition Labor alignment.

The meeting gave rise to speculation, as yet unsubstantiated, that Begin was seeking bipartisan support for a military operation in Lebanon following the slaying last Saturday in Paris of an Israeli diplomat and an increase in terrorist attacks recently in Israel and the occupied West Bank and Gaza Strip.

Bar-Lev stressed in the interview, in Hebrew, that he does not know what the government plans to do, but that he believes that limited "unorthodox" methods could be adopted without resorting to a full-scale invasion of southern Lebanon.

He said he could envisage situations in which Israel would have no choice but to attack, such as if Israeli civilians living in border settlements were unable to live in peace.

Bar-Lev said there are a number of conditions that Israel might like to change in Lebanon but they do not warrant military action now. Among them, he said, are a desire to remove the PLO and the Syrian Army from Lebanon and to have a Lebanese central government more responsive to Israel's security needs.

Bar-Lev's apparent hint, albeit oblique, at the topic of discussion at Tuesday's high-level bipartisan meeting, which has been shrouded in secrecy, went beyond statements by Labor Party leader Shimon Peres, who has said only that "questions of foreign policy and defense" were the only subjects raised.

Peres said that the slaying of Yacov Barsimantov, a second secretary in Israel's Paris embassy, was a "clear" violation of the July 24 cease-fire between Israel and the PLO.

Gur, like Bar-Lev a Labor member of the Knesset (parliament), also cautioned tonight against precipitous action in Lebanon.

"Maybe there will be a situation where Israel will have to act in southern Lebanon, but the last days have not created such a situation and do not require us to act, because the action may bring about a general confrontation also with Syria and we do not know where it will end," Gur said in a radio interview.