Secretary of State George P. Shultz expressed opposition today to an Asian-backed plan to open "indirect" negotiations aimed at a political settlement in Cambodia.

Shultz, on the first leg of a two-week Asian and Pacific trip, threw cold water on a proposal endorsed this week by former Cambodian head of state Prince Norodom Sihanouk for indirect discussions between Cambodian resistance groups and a Vietnamese delegation including elements of the Hanoi-backed Cambodian government.

"I don't think anything that has in it implicit recognition of the puppet arrangement the Vietnamese have in Cambodia is a good thing," Shultz told reporters aboard his plane. He said the Sihanouk-backed plan apparently would fit that description because of the presence of representatives of the Vietnamese-installed Heng Samrin government.

The proposal for "proximity talks," which would involve an intermediary shuttling between delegations that do not meet directly, originated several months ago with Malaysian Foreign Minister Ahmad Rithauddeen.

The original Malaysian idea was for meetings of all the Cambodian parties: the communist Khmer Rouge headed by Pol Pot and the noncommunist groups led by Sihanouk and by former premier Son Sann on the one hand, and the Vietnam-backed Heng Samrin leadership on the other.

After objections from Thailand and other Asian countries, the proposal has been amended so that the three resistance groups would negotiate with the government of Vietnam, although the Cambodians Hanoi backs would be part of the Vietnamese delegation.

Sihanouk was quoted by Thai journalists in Peking Tuesday as calling the revised plan "a good and excellent idea." Sihanouk said Thailand had worked out this "compromise" plan and that it is acceptable to the various resistance groups.

U.S. opposition will dim prospects for the proposal, which is expected to be a topic for noncommunist Southeast Asian foreign ministers meeting among themselves and with Shultz in Malaysia next week.

Although it is doubtful that Vietnam would accept the plan, Sihanouk said support by the noncommunist Asian nations would show the world that "we have good will, that we are for peace and national reconciliation of Khmer groups."

Shultz, who is expected to visit Cambodian refugee camps on the Thai-Cambodian border before traveling to Malaysia, approved a Senate amendment to the foreign assistance bill that would provide $5 million in U.S. aid to noncommunist Cambodian resistance groups to be spent at the administration's discretion. He refused comment on a House amendment that would provide $5 million in military or economic aid to the resistance to be spent at the discretion of Thailand, but he seemed distinctly unenthusiastic about the proposal.

Thailand and other Southeast Asian nations have called for U.S. military support for the noncommunist Cambodian resistance. Until now the Reagan administration has shied away from any publicly acknowledged direct aid to the resistance groups except for some humanitarian assistance.

Shultz also seemed to reject the idea of establishing a U.S. "technical office" in Hanoi to assist in identifying and processing remains of American soldiers missing in action from the Vietnam War.

"We don't plan to open anything in Hanoi" under present conditions, Shultz said.

Assistant Secretary of State Paul Wolfowitz said that Vietnam had presented some ideas for speeding the MIA accounting process but that these are "very vague." Vietnamese officials were recently quoted as saying they would like to resolve all MIA questions with the United States within two years.

On other topics Shultz said:

*The United States was "of course grateful" to Syria for its assistance in freeing 39 American hostages from a TWA jet but that "we can't forget" seven Americans still being held in Lebanon and the Navy diver killed by the hijackers.

*Iran should bring to justice the hijackers of a Kuwaiti plane last December in which two Americans were killed. Shultz declined to make any judgment about whether Iran helped bring about release of the 39 Americans held in Lebanon, saying the information was too limited.

Regarding relations with Iran generally, Shultz was cautious, saying each country has serious problems with the other.