The Soviet Union has developed and U.S. intelligence has now seen a new bomber that could hit the United States, "underlining the need to improve our continental air defense in all ways," Defense Secretary Caspar W. Weinberger said yesterday.

Weinberger said in an interview that the new Soviet long-range bomber resembles the American B1 now under construction.

The B1 is designed to drop down low when invading enemy airspace to avoid detection by radar beams.

The United States already has a force of B52 bombers that have been strengthened to fly in low against the U.S.S.R. carrying nuclear bombs. The Soviet Union presently has no comparable bomber force but instead has stressed missiles in building its land-based nuclear offense.

Development of the new Soviet aircraft would pose additional problems for American air defenses.

Weinberger would not go into how many of the B1-type planes the Soviets have built or whether they are actually in flight tests. Other sources said that U.S. spy satellites have taken pictures of the bomber, leading specialists to believe it is being flight tested but is not necessarily in full production.

"There seems to be no doubt that they do have another long-range bomber which apparently has many of the characteristics of the B1," Weinberger said. "It is new."

His disclosures about the nature of the new Soviet bomber threat come at a time when he and other Pentagon executives are under attack about the size of the fiscal 1983 defense budget. Chairman Pete V. Domenici (R-N.M.) of the Senate Budget Committee had told Weinberger a few hours before the interview that "the present defense request represents too ambitious a spending program which cannot be sustained into the future."

Weinberger said that he feels as secretary of defense that he has no choice but to stress the gravity of the Soviet military threat as part of his effort to keep Congress from slashing the new defense budget. He said the Soviet B1-type bomber is "part of the enormous, growing, continued expansion of Soviet military strength.

"We don't see any lessening of their effort," Weinberger said in a grave voice during the interview. "We see an increasing effort each year. We don't see any pause or anything that has slowed them down."

The Pentagon in a brochure last year entitled "Soviet Military Power" said a new Soviet bomber was under development. But Weinberger's comments yesterday revealed for the first time what type of aircraft it appears to be.

Trying to protect the United States against a low-flying, Soviet B1-type plane would cost billions of dollars in new radars, warning and interceptor aircraft and antiaircraft weapons. The United States in the 1960s virtually abandoned the pursuit of a foolproof bomber defense, largely because an attack by Soviet missiles looked like the main threat to this country.

Although some military specialists still believe building a new bomber defense in this missile age would be a waste of money, Weinberger flatly rejected this view.

Moscow's new bomber would be in addition to the Backfire bomber now in service. The Backfire has limited range, however, but Weinberger said he considered it a threat to the United States because it could be refueled in flight.

"We have the need to improve our continental air defense in all ways," Weinberger said in describing the Soviet B1-type and the older Backfire.

"Mending any gaps that appear in the early warning net, improving the reliability of those warnings and improving our ability to interdict attacks the Soviets might be planning" are all necessary, he said.

He added that pursuing a continental defense against missiles as well "is a very important priority with me." He said he is not ready to scrap the treaty limiting the number of ABM defending missiles that the United States and the Soviet Union could deploy, however.

"So in all ways I think we have to recognize that defense against attack is an important part of the equation," Weinberger said. President Reagan in mapping out his strategic program said that he would deploy five squadrons of F15s to intercept any invading aircraft and would buy at least six additional AWACS (Airborne Warning and Control System) planes for North American air defense.

The fiscal 1983 budget recommends spending $690 million to upgrade the nation's air defense, an amount that would almost be doubled in the following year under the Reagan blueprint. Asked about the Soviet threat to the United States from outer space, Weinberger said the U.S. current program is adequate to combat it, adding: "As we learn more about what the potential adversaries have, we may have to do more" to prepare for combat in space.