A government prosecutor said yesterday that there is "no way" to identify more than 1,500 missing FBI documents dealing with allegedly illegal government break-ins and surveillance directed at the radical Weather Underground in the early 1970s.

Despite the admission, Justice Department attorney Francis J. Martin claimed that many of the missing documents were copies of FBI documents that still exist. But Martin said that "in terms of specific documents there is no way to identify what is no longer available."

Defense attorneys for FBI former acting director L. Patrick Gray II and two of his top aides, W. Mark Felt and Edward S. Miller, contended at a day-long federal court hearing that the documents are crucial to their ability to defend the former FBI officials against charges that they authorized the surreptitious break-ins as part of an FBI effort to find Weather Underground fugitives.

The defense attorneys have asked U.S. District Court Judge William B. Bryant to dismiss the indictment against the former FBI officials because of what one defense lawyer, Thomas A. Kennelly, said was the "grossest negligence" by the government in allowing the destruction of the 1,527 documents.

Government prosecutors offered no explanation yesterday of how the documents were destroyed. It was disclosed last month that Robert Shcakelford, the former head of the FBI's internal security branch, destroyed 47 documents involved in the case.

Bryant took no action on the defense request for dismissal of the indictment. He is expected to hear more witnesses today, including Shackelford.

William L. Gardner, who headed the government's investigation of the allegedly illegal break-ins until last December, when he quit the probe in a dispute with Attorney General Griffin B. Bell, testified yesterday that the FBI files seized in the investigation in 1976 were "very important."

Gardner, now head of the Justice Department's criminal section of its civil rights division, said, however, that he believes most of the 2,680 documents taken by investigators on Aug. 19, 1976, were copies of other FBI documents. But he said he could not now recall what he believed to be the case when the raids took place.

He said that he realized that some documents no longer existed when government probers went looking for 232 files in the case in October 1976.

"We were concerned, to say the least, that material that was in the bureau's possession has been destroyed," Gardner said. "We took steps to find out why."

Asked by defense attorney Brian P. Gettings what measures he took to investigate the missing documents, Gardner replied, "That's what I can't tell you."

Gardner also testified that, despite the fact that the documents were missing, he issued no orders to insure that more documents were not destroyed.

Donald E. Moore, a retired FBI agent who worked under Miller in the FBI's intelligence division, testified that the information on the surreptitious break-ins - often called "black-bag jobs" by the FBI - would not have been placed in the FBI's permanent main files.

Rather, he said, that information and other investigatory notes would have been written up in "Do Not File" memos, many of which are among the missing 1,527 documents.

In his speech, Jackson said West Europeans had displayed "an astonishing lack of interest" in the SALT II agreements, although - he charged - they "profoundly and adversely affect European security."

The Ford and Carter administrations decided to exclude mediumrange atomic weapons based in Europe from the SALT negotiations, a decision Jackson now feels makes the new pacts dangerous for NATO.A Soviet buildup of these "theater nuclear weapons" has progressed so far, Jackson charged, that the Soviets might be able to launch "a first, disarming strike" against NATO's nuclear stockpiiles in Europe.

Jackson said the emerging SALT pacts' restrictions on the range of land-based cruise missiles will make it difficult for the West to counter Soviet force in Europe, since the land-based cruise missile would have been the logical weapon that NATO could use to match the Soviet buildup.

A protocol to the new pacts would limit the range of land-based cruise missiles to 600 miles, limit their effectiveness in Europe.