Seeking to regain initiative in the debate on SALT II, Defense Secretary Harold Brown said yesterday the administration might be willing to add additional billions to future defense budgets to meet the nation's security needs.

Testifying before the Senate Foreign Relations Committee, Brown said the administration felt the Senate's vote Tuesday for larger defense budgets in the early 1980s had "real significance," and "we will take it into account."

"The door is open in fiscal 1981 and fiscal 1982 to more than 3 percent after inflation (the increase in the defense budget in those years that the administration now favors) . . . if we conclude it is needed to carry out the major objectives of the five year defense plan," Brown told the committee.

Brown's remarks were clearly intended to signal flexibility in the 55 senators who voted Tuesday to permit 5 percent annual increases in the defense budget in those years as part of the annual budget resolution. The administration will need the support of many of those 55 if it is to have any hope of winning two-thirds approval for the strategic treaty.

However, some backers of that 5 percent proposal yesterday questioned the sincerity of Brown's new position. citing the administration's position in yesterday's House of Representatives debate on the budget resolution as evidence of possible bad faith.

In the House, the administration supportdd Rep. Robert N. Giaimo (D-Conn.). chairman of the House Budget Committee, whose compromise budget resolution calls for a reduced defense budget that falls far below the 3 percent after-inflation increase in this year's defense outlays that the administration and the Senate have both endorsed.

Administration officials said the Giaimo budget resolution was a delicate compromise that they had to respect at this time in order to get a budget resolution through the House. They promised to fight for bigger defense outlays in the House-Senate conference that will come later.

The defense outlays envisaged in the Giaimo budget resolution are close to the Carter administration's original requests for 1980, before these were increased under the pressures of SALT politics.

Sen. Ernest F. Hollings (D-S.C.), who led the Senate fight on Tuesday to raise congressionally-imposed budget ceilings to accommodate higher defense spending, complained last night that "they (the administration) are doing the same thing all over again."

He said Carter administration officials resisted his efforts in the Senate to increase defense spending and are failing to back even the 3 percent increase for fiscal 1980.

"This is making the sound of the trumpet uncertain," Hollings asserted.

A White House lobbyist defended support for Giaimo as crucial at this stage to get the budget resolution through the House. "We had a choice between making a symbolic fight in the House or working for real increases in defense spending later. We are committed to fight for the Senate's [higher] figure in the conference," the official said.

In his testimony to the Senate, Brown tried to demonstrate the administration's seriousness of purpose in the defense field. He said the 3 percent real increase in defense spending this year "is of fundamental importance to our national security."

He gave a long list of administration initiatives that he said would meet the major defense challenges of the early 1980s, including development of quick reaction forces that could intervene speedily around the world, the development of new weapons to protect American shipping, the production of new strategic weapons systems and planning for future versions, too.

Brown declined to promise that the administration would produce its 1981 defense budget or a revised five-year defense plan before January, when these would normally be released. Some senators have said they want to see these before voting on SALT II. Sen. Sam Nunn (D-Ga.) a key figure on SALT, has said he doesn't want to vote on the treaty until he has seen those two documents.

However, many senators now believe the SALT vote won't come until next year anyhow. The administration and Senate leadership say they still want a vote late this year.

In his testimony Brown sharply refuted recent statements by former secretary of state Henry A. Kissinger to the effect that West Europeans should not expect the United States to risk its own cities and territory to defend Europe against a Soviet attack. Without mentioning Kissinger, Brown declared unequivocally that the United States knew what the risks could be in threatening nuclear retaliation for an attack on Europe, but would retaliate anyway because an attack on NATO countries would directly threaten U.S. "vital interests."

Brown also said the issue of the potential "vulnerability" of American land-based missiles had been overblown, and that throughout the 1980s, the United States would retain the ability to respond flexibility and in kind to any sort of Soviet attack.

Brown concluded with the statement that without the SALT II treaty, the Senate's new interest in higher defense spending would do nothing to add to America's national security.

Washington Post staff writer George C. Wilson contributed to this article.