The U.S. Embassy here has received new samples of alleged toxic substances which, if tests prove positive, would show that Vietnamese occupation forces used chemical weapons against Cambodian guerrillas as recently as this week.

The samples were collected by members of the ousted Khmer Rouge forces in southwestern Cambodia near the Thai border and passed to the U.S. Embassy through intermediaries, a diplomat said. He said a brief note with the materials, which include leaf samples with brownish stains, said an airplane dropped the chemicals on a Khmer Rouge area Nov. 10. The note from the Khmer Rouge did not elaborate.

The samples, now sealed inside a diplomatic pouch marked with warnings against contamination, are being sent to the United States for tests, the diplomat said.

Although official Khmer Rouge battle claims and public charges of Vietnamese atrocities in Cambodia are usually given little credibility here, the diplomat said samples of chemical warfare substances collected by the Khmer Rouge have proved genuine in the past.

One such sample, he said, was among the new "physical evidence" announced by the State Department earlier this week.

The note's brief description of the alleged chemical warfare incident resembled accounts of "yellow rain," which has been reported widely used against opponents of the Vietnamese-backed government in neighboring Laos for several years.

The delivery of the latest samples followed visits to Thailand by a U.N. team of chemical warfare experts and two American medical personnel to investigate reports of chemical weapons use in Cambodia and Laos.

The U.N. investigators left for New York Tuesday after extending their stay to 10 days to look into what one said were "new cases" in the north along the Laotian border.

The group of four chemical warfare experts and four aides refused to disclose any of its findings to reporters. However, an informed Western diplomat said the team interviewed about two dozen Hmong tribesmen and Cambodians who claimed to be witnesses or victims of chemical warfare attacks. The diplomat said the team also talked to Vietnamese defectors who claimed to have seen their side using chemical weapons.

One of the defectors, a 22-year-old private in a Vietnamese signals company, has told reporters and diplomats that he saw two Soviet advisers fire a chemical weapon in western Cambodia in March 1979, wiping out not only a band of Khmer Rouge guerrillas but also an encircled Vietnamese unit.

According to the diplomat, the U.N. investigators left with some specimens of alleged chemical warfare contamination that were given to them and some alleged specimens that they collected themselves.

Testing of these samples and an invitation to visit Pakistan may delay the team's report to the United Nations, originally scheduled to be delivered in mid-December, the diplomat said. Afghan refugees in Pakistan have described chemical attacks against Moslem guerrillas resisting the Soviet occupation of their country.