Leaders of the noncommunist Cambodian resistance movement told Secretary of State George P. Shultz today that large numbers of their troops have moved back into Cambodia from Thailand as part of a new strategy of hit-and-run attacks against the Vietnamese occupation of that country.

The commanders, who were driven out of their border-area bases by a Vietnamese offensive early this year, were described by aides to Shultz as "relatively upbeat" about their military operations and prospects.

Gen. Sak Sutsakhan, commander-in-chief of the Khmer People's National Liberation Front Army of 15,000 to 20,000 men, and Prince Norodom Ranariddh, son of Cambodia's former chief of state, Norodom Sihanouk, who fields about 7,000 men, made a "nonspecific" request for direct and open U.S. aid in their meeting here with Shultz, according to U.S. sources.

State Department officials refused to comment on a Washington Post report that secret U.S. support amounting to millions of dollars has been provided to the noncommunist resistance through the Central Intelligence Agency in the past three years. The officials cited a longstanding refusal to confirm or deny reports of secret U.S. operations.

Prince Ranariddh, in a brief exchange with journalists after the meeting with Shultz, said, "We're not getting anything -- zero" in secret U.S. funds.

Shultz assured the noncommunist Cambodian leaders of U.S. backing for their cause but did not go into details, according to a U.S. participant in the meeting. Faced with demands and pending foreign aid amendments from Congress, the administration is considering direct and open economic aid to the resistance but is still resisting proposals that U.S. military aid be furnished.

A U.S. participant said the two noncommunist leaders spoke of increasing cooperation in the war zone between their forces. The U.S. official said nothing was said to Shultz about cooperation between those two and their communist ally in the anti-Vietnamese battle, the Khmer Rouge army of Pol Pot, which is about 35,000 strong.

Secretary General Prasong Soonsiri of Thailand's National Security Council said the three anti-Vietnamese factions have set up a military coordinating committee to facilitate a division of labor in their battlefield operations and are improving communications and cooperation in their attacks.

Prasong said China is providing increasing military support to noncommunist troops in an extension of its longstanding support for Pol Pot's communist forces. Nonetheless, he said there is a real need for U.S. military aid to the noncommunist groups to balance the Soviet Union's military support for Vietnam and its Cambodian allies.

Both Thai and U.S. officials said they are encouraged at growing guerrilla activity by the anti-Vietnamese forces in the interior of Cambodia. They contrasted this with the more static policies of maintaining border area bases until the guerrillas were ousted from them.

"You blow up a few ammo dumps and blow some bridges, things like that, and pretty soon you get Vietnam's attention," said a U.S. official accompanying Shultz. U.S. sources said some of the guerrillas' claims of attacks inside Cambodia have been confirmed by aerial photography.

Shultz plans to visit a Thai military camp less than three miles from the Cambodian border Tuesday, as well as villages housing Thais and Cambodians in the border area. He will be the first secretary of state to go to the border since the Vietnamese invasion of Cambodia in December 1978.

Prior to meeting the Cambodian leaders, Shultz announced that the United States will supply an additional $3 million to Thailand, on top of $5 million already being supplied, to help defray the added expenses caused by Vietnamese attacks and the pullback of Cambodian camps into Thailand from the border area.

Shultz reportedly told the Thai prime minister, Gen. Prem Tinsulanond, as well as the Cambodian military leaders, that the United States is skeptical of Vietnam's overtures -- including speeded-up return of U.S. missing in action from the Vietnam War. Shultz's aides said he told both groups, however, that Washington intends to follow up to find out whether Hanoi really is prepared to move ahead on the MIA issue.

Even if this is solved, Shultz was quoted as saying, the problem of Vietnam's "illegal occupation of Cambodia" will remain an obstacle to relations between Washington and Hanoi. Prem was said to have expressed "pleasure" at this U.S. position.

Staff writer Joanne Omang added from Washington:

The State Department said the Reagan administration "looks forward to continued rapid progress" in resolving the question of U.S. servicemen missing in action in Southeast Asia for the past decade.

The Vietnamese government last week promised to return the remains of 26 men and to provide "material evidence or information" about six more, an action which, if it happens, "would represent the largest single step forward on this issue since 1975," said spokesman Edward Djerejian.

Only eight to 10 cases per year have been resolved previously, according to State Department figures.

Vietnam's promise was "in line with" pledges for an accounting of the missing that have been made since 1982 by Vietnamese Foreign Minister Nguyen Co Thach, the spokesman said. "We appreciate this concrete demonstration of his expressed commitment."

Djerejian's conciliatory tone was in line with the nonconfrontational approach apparently being taken by both sides in the technical-level meetings on MIA questions that are now scheduled six times a year.

"We are convinced the Vietnamese have much more information than they have shared with us so far," another State Department official said, "but we have no illusions. We think a large number of MIAs will never be accounted for . . . . We only call for 'the fullest possible accounting.' "