The United States has been "sending signals" to Laos in an effort to improve relations with that Vietnamese-dominated communist country, a senior State Department official said today.

U.S. actions and intentions were made known as Secretary of State George P. Shultz arrived here to meet with officials of Thailand, which borders on Laos, and foreign ministers of the Association of Southeast Asian Nations. Many of the meetings are expected to center on the Vietnamese occupation of Cambodia, which also borders Thailand, and on ways to combat it.

A senior Shultz aide dealing with East Asian affairs, who asked not to be identified, said Laos had been informed of the U.S. interest in improved relations, which he said would depend on further action by the Laotian government to accommodate U.S. concerns.

The immediate U.S. interest, according to the official, is assistance in accounting for Americans missing in action from the Vietnam War.

A number of Americans were lost in airplane crashes and other actions in Laos, which was a secondary area of conflict.

Last September, in the first sign of willingness to improve relations, Laos permitted a group from the U.S. National League of Families of American Prisoners and Missing in South East Asia to visit the capital of Vientiane and a site where a U.S. transport plane had crashed in the war. Some wreckage and fragments of bone were found, according to U.S. officials here.

In February, Laos permitted a visit by the Defense Department's Joint Casualty Resolution Center, which investigates information on missing Americans from the Vietnam War, to explore U.S.-Laotian cooperation in this area.

In late May, the subject was also discussed by former U.S. senator S.I. Hayakawa, who spent several days in Laos as part of a trip to this area as a special U.S. emissary.

What Washington wants now, according to the senior official, is cooperation in identification of the missing on a more regular basis. Another official suggested that an arrangement similar to that with Vietnam, which has agreed to four visits per year from U.S. MIA teams, is contemplated.

On the political level, Laos last fall gave members of the U.S. Embassy staff access to more important Foreign Ministry officials in Vientiane than it had in the past.

The United States maintained its embassy in the Lao capital after the Communist takeover in 1975, but for the most part the U.S. diplomats have been ignored and isolated.

Vietnam, which maintains 40,000 to 50,000 troops in Laos, is believed to dominate the Laotian government.

An official here said it is unlikely that Laos could make gestures like those of recent months toward the United States without the acquiescence of the Vietnamese.

The Hanoi government is "generally very strong" in Laos, said the senior State Department official. In discussing Washington's desire for improved relations with Laos, the official cited the precedent of Eastern Europe, where Washington seeks to maintain better relations with several Soviet-dominated countries than with Moscow.

There is no indication of an early thaw in the U.S. position toward Vietnam. Washington has no diplomatic relations with Hanoi and has sought to maintain and even tighten the isolation of Vietnam from the noncommunist world while Vietnamese troops remain in Cambodia.

The most likely U.S. gesture to Laos, if officials decide their signals are being reciprocated, would be an exchange of ambassadors to head the embassies in each others' capitals, now headed by charges d'affaires.

Economic relations are also possible. U.S. aid to Laos is barred by law. Even without changing that, the United States could take a more positive stand toward loans and aid to Laos from U.S. allies or international financial agencies.

The State Department official emphasized that the noncommunist countries of this area, including Thailand, have been kept informed of U.S. thinking about Laos. Thailand is believed to favor improved U.S.-Laotian relations.