State Department officials have produced new physical evidence, which they describe as a "smoking gun," to support their claims that biological warfare campaigns linked to the Soviet Union are being waged in Southeast Asia with the deadly new weapon called "yellow rain."

Four weeks ago, an announcement by Secretary of State Alexander M. Haig Jr. that the State Department had its first "physical evidence" to substantiate charges of biological warfare in Southeast Asia was met with skepticism, primarily because of the nature of that evidence--a single broken leaf and a few stray green bits from another leaf.

"We now have the smoking gun," Richard Burt, the State Department's director of politico-military affairs, told the arms control subcommittee of the Senate Foreign Relations Committee yesterday.

"We now have four separate pieces of physical evidence. We may soon have more, as, I regret to say, chemical attacks have been reported in Laos and Kampuchea Cambodia within the last month," Burt said.

Even the most persistent critic of the early evidence indicated that the new material might change his opinion. Dr. Matthew Meselson, a biochemist from Harvard University, testified that he "would recommend caution on the question of whether tricothecene toxins have been used in Southeast Asia . . . although the preliminary evidence indicates that they have."

Four different poisons, called tricothecene toxins and made from a fungus, are the agents that Burt said are "weapons outlawed by mankind, weapons successfully banned from the battlefields of the industrialized world for over five decades" and which now are used, "against unsophisticated and defenseless people, in campaigns of mounting extermination which are being conducted in Laos and Kampuchea . . . ."

Burt said that the new biological weapons are indirectly linked to the Soviets in several ways:

Planes identified as Soviet AN2 crop-dusting type airplanes have been dropping the "yellow rain," particularly on the Hmong hill people of Laos.

The Soviets have many scientific papers on the subject of tricothecene toxins, including papers on the mass production of these poisons.

The Soviets have the facilities to grow the fungus that produces the poison, and the equipment to extract and purify it. "There exist, in so far as we are aware, no facilities in Southeast Asia capable of producing the mold and extracting the . . . toxins in the quantities in which they are being used," Burt said.

"There is clearly a link with the Soviet Union," he said. "We at a minimum believe the Soviets . . . could stop its use if they desire."

Some of the new evidence, water from a stagnant pond in a Cambodian village, was collected some months ago at the same time and place as the first leaf and stem sample. The water sample contained 66 parts per million of a tricothecene, deoxynivalenol, several times the lethal amount.

Two other samples of "yellow powder," or tricothecene toxins, were taken from rocks after two gas attacks in Laos. One had 150 parts per million of T2, dozens of times more than the amounts in natural outbreaks of the fungus poisoning and more than 20 times the amount needed to kill humans.

Since the fungus is known to grow in the soil, attacking the roots of plants, Dr. Chester Mirocha of the University of Minnesota testified that he did not believe the tricothecenes would be found naturally in leaf, water, or rock samples. Mirocha's laboratory identified the toxins in the samples gathered from Southeast Asia.

He added that a Laotian pilot, a defector, who flew on some of the gas attacks, and captured soldiers have bolstered the accounts of refugees. Burt also noted one detail he considered especially convincing--that when the water sample was being brought back, a man spilled some of it on his clothes and quickly came down with symptoms of tricothecene poisoning