PARIS, JULY 18 -- Bowing to congressional pressure and a deteriorating situation in the decade-long civil war in Cambodia, Secretary of State James A. Baker III announced today a major shift in U.S. policy there, dropping recognition of a three-part coalition of insurgent groups that includes the Khmer Rouge and seeking new talks with Vietnam.

In making the announcement following a meeting with Soviet Foreign Minister Eduard Shevardnadze here, Baker acknowledged that the "risks are greater" than before that the Khmer Rouge could return to power.

Vietnam's U.N. ambassador, Trinh Xian Lang, told reporters at the United Nations that his government "welcomed all efforts at preventing the return of the {Khmer Rouge} to power," adding that he hoped normalization between his country and the United States would follow.

The two countries have had no formal relations since the end of the Vietnam War 15 years ago, and Washington has worked to isolate Hanoi since the Vietnamese invaded Cambodia in 1979 to drive out the Khmer Rouge.

Recent reports from Cambodia have indicated that the Khmer Rouge guerrillas are closing in on the capital, Phnom Penh. From 1975 to 1979, when they ruled the country, the Khmer Rouge launched a radical social experiment that left up to 2 million Cambodians dead through execution, starvation, forced labor and disease.

The United States has backed a coalition made up of the Khmer Rouge, the strongest fighting unit, and two non-Communist groups under Prince Norodom Sihanouk and Son Sann, a former prime minister. But congressional disenchantment with U.S. policy has been growing, and last week a bipartisan group of senators led by Majority Leader George Mitchell (D-Maine) urged the administration to change course.

The Senate Select Committee on Intelligence voted on June 28 to cut off covert aid to Sihanouk and Son Sann, and it is not clear whether the Senate will go along with a $7 million program in overt aid approved by the House.

"It is very important, we think, to do what we can to prevent a return to power by the Khmer Rouge," Baker said.

He announced that the United States would withdraw its support for U.N. recognition of the three rebel groups' coalition as the legitimate government of Cambodia, occupying Cambodia's seat. Baker said the United States would continue to offer support for the two non-communist groups and "will be prepared to enhance our economic assistance" to Cambodia. He did not elaborate, but a senior State Department official said this could be accomplished through private organizations as well as the existing Cambodian government of Hun Sen, which is backed by Vietnam.

Baker said, "We will open a dialogue with Vietnam" about Cambodia, seeking to persuade Vietnam to use its influence over Hun Sen's government "to help us construct or create the conditions that would permit the election of a free government in Cambodia." Vietnam installed Hun Sen after the invasion in 1979..

Baker said that the withdrawal last year of Vietnamese troops from Cambodia had made it easier to begin such talks. Another set of talks is focusing on U.S. soldiers missing in action and taken prisoner of war during the Vietnam War, and Baker said the progress of these discussions, as well as those on Cambodia, "will have a lot to do with whether or not we can move toward normalization" of relations with Vietnam.

State Department officials said the talks with Vietnam are to be held at the Vietnamese mission to the United Nations.

Noting the year-long impasse in Cambodian peace talks, Baker said, "For over a year now, we've been trying to have a political dialogue" through meetings of the five permanent representatives to the U.N. Security Council. "The Khmer Rouge has succeeded in turning that political dialogue into a dialogue of the battlefield. And the fighting continues, and Cambodians continue to suffer, and Cambodians continue to die."

Baker denied that the administration would undercut Sihanouk by today's action. He noted the growing bipartisan criticism of the previous policy and said that without bipartisan approval, "it will be ever more difficult to continue to generate the funds that we need from the Congress to continue this support to the non-Communist resistance."

However, Norodom Ranariddh, Sihanouk's son and commander of his army warned the U.S. government that cutting off diplomatic support for the coalition would push its two non-communist factions "into the arms of the Chinese."

While China, which backs the Khmer Rouge, issued no formal statement, a senior U.S. administration official said the Chinese opposed the U.S. shift, particularly on the issue of the U.N. seat, arguing that the administration was "moving in the wrong direction" and that the result would be "to stiffen the spine" of Hun Sen's government.