The State Department said yesterday that "significant though preliminary" scientific data indicates that poisons called "mycotoxins" have been used as chemical weapons in Cambodia.

A news conference on the issue by Undersecretary of State Walter J. Stoessel Jr. and several unidentified government experts sought to buttress a charge made Sunday in Berlin by Secretary of State Alexander M. Haig Jr.

Neither Haig nor the other officials charged explicitly that the Soviet Union was responsible for the alleged chemical warfare, but they left the implication that this was so. A State Department document given reporters, for example, noted that "mycotoxins" are not naturally found in warm climates and that countries of Southeast Asia do not have facilities to produce them in quantity, but that "the Soviet Union, on the other hand, does have the necessary facilities to easily produce the quantities reported."

The high-profile form of the U.S. allegations, in a major speech abroad by the secretary of state and a press session for reporters and television cameras in the State Department auditorium, contrasted with the unusually guarded way in which the actual documentation was discussed.

The U.S. evidence, officials said, is based on the analysis of a single "leaf and stem sample" of chemicals found this March in Cambodia near the Thai border, presumably by Pol Pot forces of Democratic Kampuchea against whom they were used.

The press conference briefers refused to disclose the identity of the person or persons who reported that the sample contained the poisonous "mycotoxins." In an unusual display of secrecy, the government refused to tell reporters, even on a not-for-publication basis, the identity or agency affiliations of those who were answering the questions at the briefing.

Rumors and reports of injury and deaths from a "yellow rain" of chemicals in Southeast Asia date back at least to 1976, and some U.S. officials have long expressed suspicion that the Soviet Union is involved. Recently, similar reports have come from Afghanistan.

The United States, while expressing concern to the Soviets diplomatically and joining the call for a United Nations investigation, has had only circumstantial evidence in the past that chemical weapons were being used.

The Soviets have denied all charges of chemical warfare and claimed that the United States had done so in Vietnam and elsewhere.

The new findings, according to State Department officials, arose from "a very unique analysis method," not yet patented, of a person outside of government which identified toxins [poisons] of a substance called trichothecene, in the "leaf and stem sample" from Cambodia. The results from this chemical analysis have been available to the government only in the past several weeks, the officials said.

In the past, the officials said, the U.S. suspicions were directed to traditional chemical warfare agents, rather than to poisons of the "mycotoxin" group. Although there have been well-documented outbreaks of "mycotoxin" poisoning in Russia since the 1800s stemming from diseased grains and mold, U.S. intelligence did not credit the Soviet Union with producing the substance artificially, the sources said.

Stoessel, in his statement to reporters, said there are "striking" similarities between the medical effects of "mycotoxins" and symptoms reported by doctors investigating the incidents in Southeast Asia. These include dizziness, itching, blisters, nausea, coughing of blood, vomiting of blood, shock and death of those directly under the sprays.

Stoessel said the test results and medical evidence, taken together, represent "strong and compelling, but nonetheless preliminary, evidence that the lethal agents used are mycotoxins." Similarly qualified language was used throughout the reports made public by State.

Further tests are under way on additional samples of material received from Cambodia and Laos, informed officials say. It was not clear why the government chose to make public "preliminary" results based on a single field sample rather than await more conclusive data.

U.S. Ambassador Jeane J. Kirkpatrick met U.N. Secretary General Kurt Waldheim yesterday and presented a note containing U.S. information on the subject and requesting that a U.N. group investigate the reports. The United Nations voted last December to establish an impartial commission to investigate the reports of chemical warfare use. The United States also will present the information to "governments throughout the world," Stoessel said.

The United States has not made a formal complaint on the issue, as it is entitled to do under the Biological Warfare Convention. Such a complaint to Vietnam, which is believed to have dropped the "yellow rain" chemicals from aircraft, or to the Soviet Union, is reported to be under consideration.