Treasury Secretary James A. Baker III declined yesterday to endorse the newest tax proposals of Senate Finance Committee Chairman Bob Packwood (R-Ore.), but he encouraged the panel to keep working on them.

In his first meeting with senators since the panel halted voting on an earlier plan and began meeting privately, Baker said the general outline they are working on satisfies President Reagan's three main goals for tax overhaul, according to senators who were present.

"Secretary Baker expressed a great deal of interest in the proposal and urged us to go ahead. . . we are going to have to proceed without presidential endorsement," said Sen. John Heinz (R-Pa.), who added that he had "concerns" about deciding on a tax bill without any promises from Reagan he will support the result.

Packwood said after the meeting in the office of Senate Majority Leader Robert J. Dole (R-Kan.) that Baker had not expressed reservations about any particular provisions. Packwood called the meeting, he said, because Baker was about to leave to join Reagan for the economic summit in Tokyo and had not yet seen the new tax proposals.

Packwood said after the meeting with Baker that a tax package "isn't there yet," although it is "gelling."

"Gelling is one thing; concrete is another," Packwood said.

The panel found that out yesterday, as members began raising questions about the contents of the comprehensive proposals.

Senators said they continued to be pleased with the low-rate, broad-based tax scheme Packwood had proposed. But they said they were worried about some of the deductions the plans would eliminate in order to raise enough revenue to pay for the cost of reducing tax rates to a maximum of 26 percent or 27 percent for individuals.

The current top tax rate is 50 percent. The proposals would not significantly change the share of income taxes paid by different income groups because they would terminate some deductions that principally benefit upper-income taxpayers.

On the business side, the committee agreed, very tentatively, to tax increases to help pay for the lower individual rates. They included measures to bring in $11.3 billion over five years through stricter enforcement of the tax code, limits on a tax break for homebuilders called "builder bonds" and a stiffer minimum tax on business.

Discussing the general reaction of the finance panel to Packwood's proposals, Sen. David L. Boren (D-Okla.) said the panel has "a generally favorable feeling about a lower-rate approach, and a few warning flags."

Boren said he and other committee members expressed concern in the meeting -- at which no votes were taken -- about the proposed elimination of tax-deferred Individual Retirement Accounts, the termination of low tax rates for capital gains and a new provision that would not let taxpayers deduct investment losses from their income before taxes are calculated.

Packwood said after the meeting that he thinks that the committee will approve tax-overhaul legislation, even if it is less comprehensive than the current proposals.

"We will get a bill," he said. "Whether we get this bill or send out a minimum-tax bill instead, I'm sure we will get a bill of some kind."

Sen. Max Baucus (D-Mont.) was less optimistic. Asked what point the committee could be said to have reached if the chase for a tax bill were a 100-yard dash, he responded: "We're out of the blocks, but we haven't been training. We know where we are, but we don't know how far we have to go. We've gone the first 10 yards."