Hundreds of thousands of explosive devices dropped by U.S. aircraft on LAOS' Plain of Jars are continuing to kill and maim peasants who have returned there, according to three Americans who recently visited the plain.

The bombing of the Plain of Jars, probably the heaviest anywhere in Indochina, destroyed virtually all buildings, roads, irrigation canals and dams, making a return to normal life extremely slow and difficult, the Americans said. In addition, even though the bombing ended in 1973, the huge amounts of "unexploded ordance vastly complicate efforts to rebuild.

The three Americans, Linda and Murray Hiebert of the Mennonite Central Committee, and Louis Kubicka of the American Friends Service Committee, said in an interview here that Laotian authorities on the Plain were seeking international assistance to help clear the explosive devices and rebuild the irrigation works.

"Their top priorities are equipment to rebuild canals and dams and technical assistance in removing the vast amount of unexploded ordance lying all over the place," said Kubicka. "Some people we met said if the Americans had the technology to drop these bombs, they certainly must have the ability to remove them."

The visitors were told that 17 persons have been killed and 25 wounded by explosive devices in the area. They met a man who was wounded when he struck a pellet bomb with his hoe while working in his vegetable garden.

Murray Hiebert said they saw assorted explosive devices, including pressure mines disguised to resemble leaves, "Everywhere we went. Any of us could have been killed, I guess." He added, however, that most of the devices would detonate only when they were struck by something. For this reason, Kubicka said, his organization intends to supply farmers with pitchforks, which could be eased into the soil, in place of the traditional hoes, which are swung overhead.

The Mennonite and Quaker organizations represented by the Hieberts and Kubicka were outspoken opponents of U.S. participation in the Indochina war and have played an active role in reconstruction since the Communist authorities have rarely permitted Westerners to visit the Plain of Jars.

So many water buffalo, which the local people use for draft animals, have been killed that farmers frequently must hitch themselves to plows, Linda Hiebert said.

Most of the 8,083 deaths listed by the Laotians were caused by the bombing itself. Local officials also showed the visitors a detailed accounting listing 257,486 chicken and 64,529 cattle as having been killed in the bombing.

Former U.S. ambassador to Laos William H. Sullivan testified in a closed Senate hearing in October 1969 that civilan targets were never struck in the bombing of the plain of Jars. "It was the policy not to attack populated areas," Sullivan testified.

Kubicka, who has visited southern and northern Vietnam since the end of the war there and has been based in Laos since 1974, said the bomb damage and the continuing danger to humans and animals from live explosives on the plain were "by far the worst" he had witnessed anywhere.

According to various Western estimates, between 150 million and 300 million pounds of explosives were dropped on the Plain of Jars from May 1964 through September 1969. The Laotions say 600 million pounds.

The American planes, flying out of bases in Thailand, dropped a wide variety of the deadly, sophisticated white phosphorous. Fragmentation, antipersonnel bombs and immediate-and delayed-action high explosives. Aside from those killed, 3,764 persons reported were maimed.

So fearsome was the bombing of the Plain of Jars and its surrounding province, Xieng Khouang, that by the time the bombing ended, the area was deserted. The former population of 150,000 was either killed or driven out.

Since the area was captured by Communist North Vietnamese and pathet Lao forces in 1970, some 105,000 persons have returned and have begun efforts to rebuild.

Kubicka said the basic problem was a manpower shortage. "Even the Vietnamese civilian road workers we saw were just a handful. And they have almost no equipment other than head baskets and hand tools."

The three Americans visited the Plain of Jars in a party of 13 Westerners from U.N. and voluntary relief agencies. Between Nov. 16 and 19 they covered 120 miles, most of it in Soviet and Chinese-built jeeps. At one point, it took them 2 1/2 hours to drive 18 miles. The Americans said, that they saw no prewar building standing aside from a couple of plywood structures. All houses and other building they saw were made of bamboo plastered with mud.

The plain is named for approximately 100 to 200 large brownish and grayish earthenware jars, though to be funeral urns and reliably established as being 2,000 years old. The Americans said they saw one bomb crater near some jars, but otherwise the ancient vessels were unscatched.