Defense Secretary Harold Brown asserted yesterday that the newly disclosed "Stealth" technology for making warplanes invisible to Soviet radar "alters the military balance significantly."

"Any new bomber would use some elements of this technology" so it could penetrate enemy defenses, Brown added at a Pentagon news conference.

The defense secretary, who was joined at the news conference by William J. Perry, Pentagon research director, and Lt. Gen. Kelly H. Burke, Air Force research chief, said the technology works.

Some experimental Stealth planes already have been built and flight tested, although the bomber version advocated by some Air Force leaders is still on the drawing board.

"Stealth technology enables the United States to build manned and unmanned aircraft that cannot be successfully intercepted with existing air defense systems," said Brown.

It is unusual for the Pentagon to call a news conference to announce a technological breakthrough and make such broad claims for it. Here the claims come in the context of an election year in which President Carter has been under almost daily Republican attack for letting U.S. defense decline.

Brown said the Pentagon was lifting the secrecy lid on Stealth, but just a bit, because press disclosures over "the last couple of weeks" have made it "not appropriate or credible for us to deny the existence of this program."

He added that a growing number of people are learning about Stealth, that its advances are pertinent to the debate in Congress about what kind of bomber should be built for the future and that the Carter administration wanted to clear up some of that misinformation about the program.

Shortly after the Pentagon news conference allies presidential nominee Ronald Reagan mobilized for a counterattack. Aides said Sen. John Tower (R-Tex.) and William Van Cleave, military adviser to Reagan, will charge that the Pentagon's Stealth news conference was a political ploy that will end up helping the Soviets.

Although Brown and Perry would not disclose how aircraft are made invisible to radar by Stealth technology, other sources said it is a combination of factors including radar-absorbing paint and airplaine skin, fuselages shaped to minimize the number of radar beams that bounce off them, and electrical energy generated within the aircraft that deflects radar.

Industry sources said that Lockheed's "Skunk Works" at Burbank, Calif., headquarters of the celebrated Kelly Johnson, designer of the U2 and SR71 spy planes, designed one of the Stealth planes already flight tested. Several other U.S. aerospace firms are involved in the Stealth program as well.

Congress has directed the Pentagon to choose by March 15 a bomber design for the future and have one flying by 1987. Perry said yesterday that if the Stealth bomber is recommended at that time or before, it will be up to Congress to adjust the deadline as needed.

The rave notices Brown and Perry gave yesterday to the penetrating capabilities of a Stealth airplane would seem to give it the edge over such other candidates as a modified B1 bomber, a stretched Fb111 or a simple but vulnerable cruise missile carrier.

One source said yesterday that the Stealth bomber on the drawing board would be a high-flying one, like the B70 President Kennedy canceled, instead of a low-level penetrator like the B1.

Not only does Stealth promise to protect warplanes, said Perry, but "this technology could be applied to any military vehicle which can be attacked by radar-directed fire."

The research director said, "We are considering all such applications."

Brown said Stealth did not figure in Carter's 1977 decision to cancel the B1 bomber in favor of cruise missiles.

While stating that "none of the most sensitive and significant" secrets about Stealth have leaked out, Brown predicted the Soviets will step up efforts to improve their air defenses to cope with this new threat.

Stealth, said Brown, "promises to add a unique dimension to our tactical forces and the deterrent strength of our strategic forces . . . It can contribute to the maintenance of peace by posing a new and significant offset to the Soviet Union's attempt to gain military ascendancy by weight of numbers.'