JERUSALEM, AUG. 12 -- The United States today reinforced its call for Israel to scrap the disputed program to build its own fighter plane.

In Washington yesterday the State Department publicly criticized the project. It stiffened that message today with a private one from Secretary of State George P. Shultz, delivered by his aide to several Cabinet ministers here, an Israeli official said.

Both messages, delivered only days before a scheduled Cabinet vote on the planned Lavi warplane, said that neither Israel nor the United States, which funds most of the project, can afford it. Israeli leaders said tonight that they are weighing the U.S. advice but stopped short of promising to kill the Lavi.

A U.S. Embassy spokesman would not confirm the private message, although an Israeli official said it had been delivered by a senior embassy officer and repeated to Prime Minister Yitzhak Shamir by Shultz's visiting aide, Charles Hill. The official said the Reagan administration reaffirmed a commitment made several weeks ago to offer alternatives to the Lavi.

The Lavi was proposed in the late 1970s as a small, inexpensive fighter but grew into a plane high in performance, technology and cost that is now proving politically difficult to kill. Top defense and economic officials agree that the plane, with an estimated cost of $18 million apiece, will drain resources from other vital projects and actually risk weakening Israel's military security.

But labor unions and the influential, state-run Israel Aircraft Industries have protested a loss of contracts and jobs if the Lavi is canceled. Their pressure, and considerable national pride invested in the Lavi, have persuaded Shamir to continue his support for the project and Foreign Minister Shimon Peres to lean in its favor.

Shamir and other officials said Washington had attached no threat to its advice but had simply pointed out the limitations on U.S. aid. Interviewed on Israeli television, Shamir said, "I think we will . . . decide on it {at a Cabinet meeting Sunday} and the decision will be Israeli."

Peres and Defense Minister Yitzhak Rabin echoed other officials in acknowledging that the United States could advise Israel on the matter since it pays more than 90 percent of the Lavi's costs.

But analysts suggested they glimpsed discreet carrots and sticks in the U.S. stance. Today's U.S. message reaffirmed assurances given to Rabin in Washington last month, an Israeli official said. Israeli and U.S. observers said those assurances include a promise to allow more of Israel's $1.8 billion economic aid package to be spent in Israel, where it might help create jobs to replace those lost with the Lavi.

Israeli press reports on yesterday's declaration in Washington cited fears that U.S.-funded military subcontracting work in Israel would diminish if the Lavi is kept alive, and that long-term military cooperation would suffer.

The Israeli Cabinet is reported to be split on the issue, and the U.S. pressure coincides with a vigorous campaign by the Lavi's opponents to kill the plane.

The sensitivity of the issue seemed reflected last Sunday in a vote on the Lavi in the parliament's foreign affairs and defense committees. Fewer than half of the members showed up for the joint vote, leaving a minority of conservative legislators to approve the project's continuation.