Secretary of State James A. Baker III told Congress yesterday that, in an effort to resolve the 11-year war in Cambodia, the United States has agreed to begin direct talks with the government installed there by Vietnam.

Baker told the Senate Foreign Relations Committee that Cambodian Prime Minister Hun Sen had taken "what appears to be a constructive attitude" toward a plan agreed upon last week by the five permanent members of the U.N. Security Council to have the United Nations administer Cambodia until elections are held.

"Assuming that there is a continuation" of that attitude and "in order to try to support this process as much as we can," Baker said, "we have decided that we will take the next logical step now, and we will begin a dialogue with Hun Sen's representatives through our charge" in Vientiane, Laos.

Baker's announcement came as the four Cambodian factions gathered in Jakarta, Indonesia, for a critical meeting on whether to accept the peace plan adopted by the United States, the Soviet Union, China, Britain and France.

Analysts said the announcement, which lends substantial legitimacy to Hun Sen, might encourage him to accept what, under the plan approved last week, would be a major relinquishing of power to the United Nations and a Supreme National Council comprising representatives of his government and the three-party guerrilla coalition opposing him.

The precise composition of that National Council, which would include representatives of two non-communist groups and the radically communist Khmer Rouge, has been the focus of recent negotiations.

It also might encourage Hun Sen to attend the Jakarta talks, analysts said. Hun Sen said last week that he would not go unless Prince Norodom Sihanouk, head of one non-communist faction and nominal leader of the resistance coalition, also attended. Sihanouk has said he would not.

Their absence has led to concern that the talks may be scrapped.

Baker had proposed direct talks with Hun Sen in July when the administration, in a dramatic policy shift, announced that it would withdraw recognition of the three-part coalition and would talk about Cambodia with Vietnam, the Hun Sen regime's key backer.

At the time, however, President Bush declined to have direct talks with Hun Sen, preferring to test his reaction to the policy changes, sources said. Bush changed his mind earlier this week, a senior U.S. official said.

Baker said yesterday that "even though Hun Sen has expressed disagreement with some details" of the plan adopted last week, "it is my understanding that he has agreed to use it at least as a basis for discussion with the other factions," adding that there was "some prospect of real progress" toward ending the fighting.

Sources said the administration's policy shift in July was a bow to erosion of congressional support for continued aid to two non-communist resistance groups allied with the Khmer Rouge, who are believed responsible for the deaths of more than 1 million Cambodians from 1975 until Vietnam invaded in 1978.

Congressional critics of U.S. policy at the time said the shift was not enough and called for further changes, including direct contacts with Hun Sen. Senate Majority Leader George J. Mitchell (D-Maine), a key critic of the policy, said yesterday that he was "pleased with Baker's announcement."

Rep. Chester G. Atkins (D-Mass.), another leading critic, said that he, too, was pleased but that the latest stance was "tepid." He called for direct talks at a higher level, adding, "I am hopeful, however, that the anouncement sends a message to {Sihanouk} that our own efforts to reach a more evenhanded policy cannot be derailed."

Under the plan approved by the Security Council members last week, key government ministries would be controlled by the Cambodian National Council and thousands of U.N. administrators and peace-keeping troops. The transitional arrangements would also include disarmament of the rebel groups.

Critics of the plan, which would be by far the U.N.'s most ambitious undertaking, have said that the cost could exceed $5 billion and that no one has committed to funding the plan.