Vietnam's handpicked government in Cambodia agreed to open the Mekong River for the transportation of relief supplies as the international community appealed here today for strong action to save millions from starvation.

The gesture from Phnom Penh, disclosed in a statement distributed by Vietnam's U.N. mission, was welcomed as a step in the right direction by American officials who said at the same time that further steps are required.

The Phnom Penh authorities and their Vietnamese backers, however, were still resisting the main U.S. plan for speedy and large-scale action to relieve famine -- a "land bridge" of truck convoys operating from Thailand.

Unlike the proposed truck convoys across militarily contested areas, ships and barges up the Mekong to Phnom Penh could be easily controlled by the Vietnamese forces.

Secretary of State Cyrus Vance, formally announcing a U.S. relief commitment of $69 million to the U.N sponsored "pledging conference" for Cambodia, called for pursuit of "every avenue" and "every means" to bring in food and medicine whether by land, sea or air.

Among the ideas proposed here in a long day of money pledges, oratory and corridor discussions were air drops of food from U.N.-sponsored transport planes and the opening of several auxiliary airports in outlying parts of Cambodia.

The one-day conference involving 76 nations was called to raise and announce pledges of funds and demonstrate broad political support for internationally sponsored relief efforts.

The meeting raised $210 million in cash and supplies, about two-thirds of the goal of $310 million for the next 12 months. U.N. Secretary General Kurt Waldheim hailed the results as providing "the financial resources and the moral commitments" necessary for an urgent and adequate relief program.

One after another ranking diplomats of noncommunist nations described the Cambodian tragedy in apocalyptic terms, while appealing for dramatic action.

"A country's people is dying in Asia. We are here to try to save them," said French Foreign Minister Jean Francois-Poncet, one of the moving figures behind the U.N. conference.

"The entire Khmer people of Cambodia are faced with annihilation through famine and warfare . . . . Unless we help, Mr. Chairman, they will nearly all die," declared Canada's Secretary of State for External Affairs Flora MacDonald.

Vietnam's U.N. representative. Ha Van Lau, on the hand, told separate meetings of U.S. senators and American journalists that famine reports from the scene are "exaggerated" in his personal view.

Lau also met with Vance, his old negotiating partner from the U.S.-Vietnam peace talks in Paris a decade ago. Afterward, Vance reported no progress, at this moment, toward Vietnamese approval of the truck convoys.

American officials estimated that a Mekong River transportation route could bring as much as 8,000 tons of supplies a month to Phnom Penh when it is fully operational. In addition to the improvement of existing air and sea routes, this could contribute substantially to the proposed supply of about 30,000 tons of food per month, roughly 1,000 tons per day, which international groups believe is required to feed hungry Cambodians.

The Soviet Union, the superpower patron of Vietnam and its Phnom Penh regime, did not pledge additional aid at the conference but announced pointedly that it has been helping "in deeds rather than words."

Ambassador Oleg Troyanovsky said Moscow already has provided large amounts of aid, including 159,000 tons of rice, corn and other grains, 660 vehicles, 7,800 tons of "rolled iron articles," 903 tons of paper and $5.5 million worth of "household dishware."

Nations that are "genuinely interested" in aiding the Cambodian people should do so only through the Hanoi-sponsored "People's Revolutionary Council" in Phnom Penh, Troyanovsky said.

The United States is considering new appeals to the Soviets and Chinese for cooperation with humanitarian efforts in Cambodia. In this pursuit, Vance may meet as early as Tuesday with Soviet Ambassador to Washington Anatoliy Dobrynin, State Department officials said.

The sponsors of today's conference made strenuous efforts to avoid the Moscow-Peking rivalry that underlies the military struggle in Cambodia and to skirt other politically sensitive aspects of the situation.

Representatives of both claimants to power in Cambodia, the Vietnam-backed Heng Samrin authority in Phnom Penh and the Chinese-backed Pol Pot forces, were present today, but neither was given the floor to speak. p

With U.S. backing, the Pol Pot forces are still recognized by the United Nations as the legitimate government of Cambodia. The United States provided visas for two Heng Samrin representatives at their request in the interest of the humanitarian appeal, American officials said.

After a 90-minute meeting with Ambassador Keo Prasath, who represents the Heng Samrin government in Moscow, Sen. James Sasser (D-Tenn.) said he was encouraged at the diplomat's admission of a serious famine in Cambodia and his relatively forthcoming position on the supply of food and medicine.

While Phom Penh would not accept the truck convoys proposed by Washington, Keo Prasath said the Cambodians would welcome an increase in international aid and would agree to the supervision of its distribution by UNICEF and the international Red Cross, according to Sasser.

Canada's MacDonald made one of the few direct references to "the root causes of this appalling situation" in her appeal for a solution to the underlying political problems involved. "The Vietnamese government, along with the Heng Samrin and Pol Pot authorities, must stop playing politics with the very lives of millions of people," she declared.