A senior U.S. diplomat met throughout the day today with members of Nicaragua's guerrillia-backed provisional junta in Costa Rica in an abrupt departure from previous U.S. policy of staying out of direct negotiations with the group.

Meanwhile, U.S. Agency for International Development disaster experts flown here this week to assess emergency assistance needs said they plan to begin immediate U.S. Air Force flights of 45 tons of food daily to avert possible mass starvation in warton Nicaragua.

The United States continues to maintain that President Anastasio Somoza's resignation is imminent, perhaps this week, that a negotiated agreement with the junta over a future government will be reached and that the six-week-old civil war will end.

While Carter administration envoy William Bowdler was conferring with the guerrillia-supported junta, two conservation U.S. congressmen arrived here today in a show of support for Somoza and his government.

Somoza has said he will go when the United States assures him that what it considers a non-Marxist government will take over and that his army, the Nicaraguan National Guard, will be preserved. Over the past several weeks, first the United States and more recently other Latin American governments have tried to pressure the junta to expand its membership with political moderates and to guarantee the perpetuation of the National Guard as an institution.

Thus far, the junta has strongly resisted on both counts, despite international threats that support of Sandinista National Liberation Front forces will be cut off.

The junta shares a widespread Nicaraguan antipathy toward historical U.S. intervention here. Despite U.S. efforts to spare its sensibilities by allowing Venezuela and Costa Rica to negotiate with the guerrialla-backed group, lack of progress and continued rebel military advances apparently spurred the United States to become directed involved.

Bowdler arrived in Costa Rica last night from Panama and went immediately into meetings with the junta that were resumed this morning and were still going on by late afternoon.

In a telephone interview this morning before today's meeting began, Nicaraguan businessman Alfonso Rovelo, one of five junta members said he saw "no reason why the United States should lay down conditions on how we should run Nicaragua."

In addition to addng members and guarantees for the National Guard, he said, a 10-point U.S. plan included provisions on human rights and revised judicial and electoral systems -- all of which were included in a junta program released two weeks ago.

"It irritates us to be told to do things we've already said we'd do," Revelo said. At the same time, "we see no logic to broadening the junta when it has been backed" by Nicaragua's largest moderate opposition political and business groups.

As for the National Guard, Revelo said, "We're fighting to eliminate Somoza -- and they're trying to maintain parts of the National Guard."

Informed sources said the United States believes that without more moderate members, the current junta balance of one Sandinista, one hardline socialist, a center left academic and two moderates would be unable to attract financial support from the conservative international economic community. It also believes that without the National Guard to act as a police force preserving public order, radical Sandinista elements would try to take over the country.

The junta feels its government is both representative and sufficiently moderate. It believes the National Guard is so hated in Nicaragua that while "many" of its members could be integrated into a new national army, any attempt to preserve it as an institution would be opposed by the majority of the people and rejected outright by the guerrillas.

But the junta recognizes it must deal with the United States as the dominant foreign power both in Nicaragua and throughout the hemisphere. Although the pressure for concessions began to come from Costa Rica and Venezuela last week, the junta has always considered the proposals to be U.S. motivated.

That feeling apparently struck a chord with the United States, which previously had denied taking a dominant role in the negotiations, and led to today's meeting with Bowdler. By early evening, however, there was no indication of the outcome of the session.

U.S. Reps. George Hansen (R-Idaho) and Larry MacDonald (D-Ga.) spent several hours here today. The purpose of their visit was to deliver cartons of food and medical aid and to show support for the Somoza government.

The conservative congressmen met with Somoza and toured war destruction and refugee camps in the cities before meeting with members of the government's Liberal Party. Both congressmen blamed the lack of U.S. support for the Somoza regime on "distorted press accounts of Nicaragua and a failure to respond to threats from international Communists."

"Take heart, keep up your courage and stand fast," MacDonald told the party. "The American people . . . are beginning to understand your plea."

In an emotional exchange before the meeting with reports from ABC television network, whose correspondent Bill Stewart was killed here by the National Guard June 20, MacDonald said there was "no question of the distortion of news (of Nicaragua) in the United States."

An ABC reporter asked MacDonald to "walk down here on the beach at the lake and look at the bodies with their hands tied that (the National Guard) is burning in the morning. And you come down here and accuse us of distorting? Where the hell have you been, sir?" CAPTION: Picture, Finger on the trigger, a Sandinista rebel guards a barricade at the Nicaraguan town of Jinotepe. UPI