JERUSALEM, FEB. 5 -- With Iraqi missile attacks on Israel diminishing, Prime Minister Yitzhak Shamir is left with a crucial political decision: whether, and how, to satisfy strong leadership pressure for an Israeli counterblow against Iraq.

In a letter to President Bush leaked to the Israeli press today, Shamir said an Iraqi attack with chemical weapons or one causing heavy casualties would "create an intolerable situation that will require an immediate response on our part."

Official sources confirmed the published accounts of the message and said that with it, Shamir had in effect defined the point beyond which Israel would definitely not agree to further restraint in retaliating.

Even without such provocation, however, Israeli military leaders appear eager to act. In a television interview Sunday night, Deputy Chief of Staff Ehud Barak said Israel's army "has very good operational plans to deal with removing the threat of ground-to-ground missiles from western Iraq."

He then hinted that Israel might act without obtaining U.S. agreement. "In order to carry out these plans, coordination with the United States is preferable," said Barak, who had just returned from consultations with Pentagon officials in Washington. "But I am sure that any intelligent person in the world understands that in some situations, Israel, based on its right to self-defense, will want to act, while notifying those parties that need to be notified."

Israeli officials say the government still intends to coordinate any military operation with U.S. forces, but some added that Barak's statement reflected frustration with the refusal of the Bush administration to agree to Israeli action.

While acknowledging that U.S. and allied operations have significantly decreased the threat of Scud attacks, Israeli officials argue that the danger of more Iraqi missile strikes with conventional or even chemical warheads is still real, and thus justifies an Israeli operation.

At the same time, some officials acknowledge that there is a deeper political issue. Throughout its history, Israel's defense doctrine has been based on the principle that no Arab act of aggression against the Jewish state will go unpunished. Despite the extraordinary circumstances of the Persian Gulf War, in which Israel's enemy is being attacked by a U.S.-led alliance that includes Arab states, most senior officials argue that the tradition of retaliation should not be allowed to lapse.

"We have to think about what other Arab leaders will think in the future if they remember there was an attack on Tel Aviv and Israel did not act," a government official said.

U.S. officials and American Jewish leaders, who also largely oppose Israeli entry into the war, have argued to Shamir that the Israeli public overwhelmingly supports his government's restraint. "I don't think the Israeli people are going to be concerned that Israel take an action of its own against Iraq, as long as the end objective is achieved" of destroying President Saddam Hussein's military power, said Kenneth Schiner, international president of B'nai B'rith, who is visiting here.

American Jewish leaders, like Israeli diplomats here, are anxious to preserve the political credit Israel has won for enduring nine Iraqi missile attacks without responding.

But an aide to Shamir said bluntly that "this has nothing to do with the Israeli public. This is something that the leadership of the Israeli army and the government resent and feel very, very deeply."

"Our whole conception of ourselves is that no one will be able to attack us and go unpunished," the official said of Israel's political and military elite. "And we feel strongly that to do nothing is not healthy for us in the long run."