American travelers, curious sightseers as well as scholars, are trickling into the once war-torn nation of Vietnam, where they apparently are finding a friendly welcome. The U.S. government doesn't prohibit Americans from touring Vietnam; however, it has imposed certain restrictions that make getting there complicated.

With certain exceptions, Americans must make travel arrangements to Vietnam through a foreign travel agency based outside this country. U.S. tour operators and travel agencies are banned by law from organizing, leading or even promoting tours to Vietnam -- although there is hope in the U.S. travel industry that these restrictions will be lifted as relations between the two former belligerents continue to improve.

Though travel to Vietnam is permitted by both governments, the U.S. State Department officially discourages it, because the United States does not maintain diplomatic or consular relations with Vietnam. Nor does a third country represent U.S. interests there. If an American traveler gets into difficulty, says the Bureau of Consular Affairs, the government "is not in a position to accord normal consular protective services." In other words, you are pretty much on your own in Vietnam.

The State Department says U.S. citizens "may be placed under surveillance by Vietnamese security agencies who may be suspicious of their activities simply because they are Americans ... . Engaging in activities deemed suspicious by Vietnamese security authorities could lead to the arrest and detention of both the

American traveler and his or her Vietnamese contacts, relatives or friends."

Still, the diplomatic standoff has not detered adventurous Americans, a number of whom are veterans of the Vietnam War.

With special permission, the University of Puget Sound in Tacoma, Wash., is taking a group of 35 undergraduates to Vietnam for most of February. Similarly, Earthwatch, a Massachusetts research organization that accepts paying volunteers in its programs, will lead an officially sanctioned American study group to Vietnam in March to help save the migratory cranes of the Mekong Delta.

Americans make up about 30 percent of the clientele of Orbitours, an Australian tour organizer in Sydney that offers frequent escorted tours to Vietnam, including a 10-day rail trip. Canada Swan International Travel of Vancouver and Adventures in Paradise, a Thai company with offices in New York and Bangkok, also cater to Americans interested in exploring Vietnam and its postwar recovery.

An excellent -- and hefty -- new guide to Vietnam, "The Vietnam Guidebook" by Barbara Cohen (HarperCollins, 464 pages, $16.95), provides practical travel information as well as comprehensive background on Vietnamese history and culture. Cohen, a Northern California psychiatrist, served as a major with the U.S. Army Medical Corps in Vietnam in 1970 and 1971 and has returned to Vietnam frequently. Other publishers also reportedly are preparing Vietnam guides for next spring.

Elsewhere in Asia, other former combat areas are attracting American travelers:

Cambodia. Currently, travel into Cambodia is limited primarily to escorted tours to the ancient ruins of Angkor Wat. Americans may join these tours. As in the case of Vietnam, you must book a tour through a foreign tour operator. Most depart from Bangkok. The U.S. government does not currently maintain diplomatic relations with Cambodia or have third-country representation, and travel to the country is discouraged.

For years, Cambodia has been embroiled in factional strife, although international efforts are underway to help reach a peaceful settlement between the contending forces seeking control of the country. Meanwhile, the State Department is officially warning travelers that some popular tourist destinations -- including areas outside Angkor Wat -- are considered unsafe. Nevertheless, tour operators continue to schedule visits to Angkor Wat.

Cohen's guide includes a chapter on Angkor Wat. She currently is working on guides to Cambodia and Laos.

Laos. Laos has a Communist government with close ties to Vietnam. However, the United States maintains full diplomatic relations with Laos, and the State Department lists no travel advisories currently in effect. U.S. travel firms are allowed to arrange trips to Laos, and air excursions depart regularly from Bangkok. The capital city of Vientiane and the former royal capital of Luang Prabang, with its array of 16th-century temples, are the primary attractions.

North Korea. As with Vietnam and Cambodia, the U.S. government does not prohibit travel to North Korea, but it officially discourages it because the United States does not have diplomatic relations or third-country representation. However, American travelers may find it difficult to obtain an entry visa, which must be obtained in a country which has a North Korean embassy -- such as the Soviet Union or China.

About a year ago, Richard Liu, who heads Canada Swan International Travel of Vancouver, announced a series of escorted tours to North Korea. They were aimed primarily at Americans and Canadians, and Liu believed them to be the first such escorted tours to the Communist-led country in 40 years. He held what he believed to be an exclusive contract with North Korea's government.

A couple of trips were completed successfully, but in June of this year, Liu says, the North Korean government said it would no longer issue visas to Americans. Liu canceled his North Korean tours, at least for the time being. As far as he knows, no one else is offering tours to North Korea.

Americans who go to Vietnam, Cambodia or North Korea may return home with no more than $100 worth of handicrafts or other merchandise purchased while in each of these countries. The items must be for personal use only, and they can only be brought into the United States as accompanied baggage. Items purchased for personal consumption while abroad are not subject to the $100 limitation. In either case, you cannot use any U.S. credit cards, since they are prohibited while traveling in these countries.

These restrictions are set forth in U.S. regulations administered by the Office of Foreign Assets Control of the Treasury Department. The same regulations, which at times have been aggressively enforced, prohibit U.S. travel firms from engaging in tourism to any of the three countries.

Adventures in Paradise, a Bangkok-based travel firm with an office in New York, is attempting to negotiate its way through the rules. It recently announced a series of 16 nine-day tours to Laos and Cambodia from Bangkok to begin Nov. 15. As explained by one of the firm's American partners, Ken Fish, interested travelers can phone the New York office to obtain preliminary information -- but only verbally. Fish will forward your name and address to Bangkok, and his Thai partner will mail out an information package.

Fish says he can make travel arrangements to Bangkok and Laos, but any side trips to Vietnam and Cambodia must be handled by the Bangkok office. "In a year or so, the situation may change," he says. "We're headed toward a normalization of relations."

Fish advises Americans interested in travel to Vietnam or Cambodia to begin planning well in advance -- at least five weeks. Like other companies, his firm will assist in obtaining visas, which are required for both countries as well as Laos. The visa applications are handled through the Bangkok office.

Typically, the firm's New York office will forward to Bangkok the necessary details, such as name, age and address. The Bangkok office then will obtain advance approval for the visa. While you arrive in Bangkok, you take your U.S. passport to the Vietnam or Laotian embassies for the actual visa. A visa for Cambodia must be obtained at a stopover in Vientiane in Laos, where flights to Angkor Wat originate. To obtain a visa to any of the three countries, you must have confirmed space on an authorized tour, Fish says, whether you are going in a group or as an individual.

Tour packages to Vietnam can be obtained from private tour organizers, such as Fish's firm, or from the Vietnamese government agency, Vietnam Tourism, according to Cohen's guidebook. A complete package, she says, will include airport pick-ups, hotel transfers, all domestic travel, guide and interpreter services and admission to cultural events. The preferred mode of payment is U.S. dollars in cash on arrival.

There is "a substantial demand" for travel to Vietnam, Fish says, as well as to Cambodia and Laos. His tours attract culturally oriented Americans, academics and experienced travelers looking for places off the beaten track. They must have something of an adventurous spirit and be willing to accept occasional setbacks. For example, the tour's selected hotels usually have air-conditioning, but the manager may shut it off for the day. "And flights may be canceled for any number of reasons," Fish adds.

The food "is great," he says, and lodgings are "modest but adequate. There's nothing in the way of luxurious accommodations. If your idea of roughing it is going without room service, then this trip is not for you."

In her guide, Cohen notes that some 30 cities, towns and districts have been opened to foreign tourists. These towns have "at least minimal lodging facilities and a government tourist office."

Certain limits are imposed on travelers, she says, "partly for the safety of the visitor, and partly for government security reasons ... . Suspicion and hostility toward government agents and foreigners (no matter how friendly) still exist in rural areas of Vietnam."

No foreign traveler will be allowed "to venture unescorted beyond urban limits," she writes. Foreigners generally are forbidden to enter private homes, and Vietnamese are not allowed in hotel rooms.

In a telephone interview, Cohen said Americans in an escorted tour "will have no problem." But Vietnamese officials continue to look with suspicion on Americans traveling on their own.

For the latest travel advisories issued by the State Department, contact the Citizens Emergency Center, 202-647-5225.

Among the travel firms and educational organizations offering tours to Vietnam, Cambodia and Laos:

Adventures in Paradise. Beginning Nov. 15, this Thai firm (with American partners) is offering 16 weekly departures to Cambodia, including an extended visit to the Angkor Wat ruins. The nine-day trip includes a four-night stay in Siem Reap, about four miles from Angkor Wat, and three nights in Phnom Penh, the Cambodian capital. One night is spent in Vientiane, the Laotian capital. The all-inclusive price from Bangkok is $2,895 per person (based on double occupancy). Air fare to Bangkok is additional.

The firm also offers a seven-day independent or group tour of Vietnam, departing daily from Bangkok throughout the year. The itinerary includes Ho Chi Minh City (Saigon), Danang, Hue and Hanoi. The price is $1,425 per person (double). A one-week independent or group tour of Laos, including visits to Vientiane and Luang Prabang as well as mountain villages, is $2,100 per person (double). Air fare to Bangkok is additional for both packages.

For information: Adventures in Paradise, 155 W. 68th St., Suite 525, New York, N.Y. 10023, 1-800-736-8187 and 212-595-5782.

Orbitours. An Australian company, it has been offering tours to Vietnam and Laos since 1980 and to Angkor Wat in Cambodia since 1984. The firm's programs range from a three-day visit to Vientiane for $198 (excluding air fare) to a 24-day extensive tour of Vietnam, Cambodia and Laos for $3,750, which includes all expenses from Bangkok. Departures for the extended tour are Nov. 5, Dec. 17, April 1 and July 1.

A 10-day rail tour aboard the "Reunification Express" begins in Hanoi and concludes in Ho Chi Minh City with stops in Hue, Danang and Nha Trang. The land price for a group tour is $1,281 per person (double) plus $330 for air fare from Bangkok to Vietnam and back. Air fare from the United States is additional. A solo traveler can purchase the same tour for $1,820 plus $330 for air fare.

For information: Orbitours Ltd., 7th Floor Dymocks Building, 428 George St., Sydney 2000 (GPO Box 3309), Australia, 011-61-2-221- 7322. Because of the 14-hour time difference between Washington and Sydney, you may want to contact the firm by fax at 011-61-2-221-7425.

Canada Swan International Travel. Canada Swan offers a variety of tour programs out of Bangkok to Vietnam, Cambodia, Laos and Burma. When possible, it expects to resume tours to North Korea. For information: Canada Swan International Travel, 5701 Granville St., Vancouver, B.C. V6M 4J7, Canada, 604-266-3300.

Earthwatch. Founded in 1971, Earthwatch, an American nonprofit organization, sponsors of scientific research projects around the world. The projects are led by archaeologists and others whose work is funded by paying volunteers accompanying them for two or three weeks. Among the projects is a study of migratory cranes in the Mekong Delta of Vietnam.

Participants will assist in studying the habits of cranes in a large wetlands area outside Ho Chi Minh City. Pressure is growing to farm the wetlands, says Earthwatch, and scientists are looking for a solution to the competing demands for the land. Volunteers stay in dormitories in the town of Tam Nong, which has 3,000 residents. They take boats to dikes or blinds to record the feeding habits of the cranes.

The program runs from March 18 to April 4. The land cost is $2,240. Air fare is additional. For information: Earthwatch, P.O. Box 403, Watertown, Mass. 02272, 617-926-8200.