President Carter, in a restrained response to his first international incident involving U.S. military forces, said yesterday that an Army cargo helicopter shot down in North Korea has strayed into Communist territory by mistake.

"Our primary interest is in having the incident not escalate into a confrontation and also to account for the crew members," Carter told a group of senators.

Setting forth the known facts with no display of anger or emotion. Carter called for an early meeting with North Korean liaison officers at Panmuniom "to discuss the whole incident" and facilitate the return of an injured crewman and the bodies of the three men killed.

Unlike the killing of two American officers by North Koreans in the demilitarized zone in August, there were no high-level charges of "murder" yesterday, no threatening U.S. military deployments in the Pacific and only a minor alert of U.S forces in Korea. White House press secretary Jody Powell reported that Carter departed little from his previously planned schedule for the day, and said the shooting had not caused a crisis in the usual meaning of the word.

While some details are still ambiguous or unknown, the main lines of the incident seemed clear from the statements of White House and Defense Department officials:

The U.S. Chinook, a big two-motor craft used for cargo and transport missions, took off Thursday morning (Wednesday night in Washington) from a military base at Pyong-Tack, about 35 miles south of Seoul on the west coast of South Korea. Manned by a crew of four Americans, it was assigned to transport material to aid in the construction of a South Korean observation post in mountainous terrain just south of the demilitarized zone.

The pilot, who had never flown in the area of the DMZ before, planned to pick up a load of cement and a South Korean navigator familiar with the terrain. However, the copter never made the planned landing at Pangnung, on the east coast about 15 miles south of the DMZ.

"The aircraft apparently veered north at the eastern end of the DMZ at the Pangnung area," Powell said. Carter said the U.S. assumption is that the helicopter lost its navigation fix.

As it approached the DMZ heading toward North Korea, warning shots were fired by South Korean troops seeking to alert the copter the impending danger."Apparently the North Koreans have orders to shoot down anything that enters that territory, and the South Koreans knew it," Carter said.

Across the dividing line between the two Koreas, the helicopter was the target of North Korean fire. It is not clear whether the craft was hit then, as first reports said, but it is clear that the helicopter landed in North Korea. The exact spot is disputed, but a White House report indicated it was on a sandbar in the Nam-Gang River about 2.5 miles north of the DMZ.

Some of the crew dismounted, possibly to inspect the craft for damage. A North Korean unit was approaching, and according to some unofficial reports was seeking to inspect the helicopter. The Americans, who were reported to be unarmed, as was the aircraft, took off after one or two minutes on the ground and headed south.

At this point North Korean machine guns opened up on the copter, which crashed close to the point where it had landed. Carter said two crewmen were killed in the crash and another was apparently killed "in the exchange of gunfire in some way," Powell said later there was no firing from the Americans.

The fourth crew member was wounded. North Korea said in a message through Panmunjom that he is receiving medical treatment.

Some or all of the shooting incident was seen by two command posts on the South Korean side of the DMZ several miles away. Radio transmissions from the helicopter also spread the alarm.

Within minutes, the White House was notified through the national military communications network. Word was passed to Carter as he sat on the South Lawn of the White House with West German Chancellor Helmut Schmidt watching a performance of "Carousel" in the guest's honor by a group of Metropolitan Opera stars.

National Security Affairs Adviser Zbigniew Brzezinski went to the White House Situation Room which is loaded with communications gear and is the site of crisis meetings. He was joined by Secretary of State Cyrus R. Vance and Secretary of Defense Harold Brown, who also had been attending the state dinner for Schmidt. Carter came in around midnight after saying goodbye to his guest.

A U.S. Army infantry company of about 180 men, and 12 helicopters and 12 F-4 Phantom jet fighters based in South Korea were placed on combat alert shortly after the shooting, the Pentagon said yesterday. The alert was not announced at the time and was cancelled a few hours later.

Press secretary Powell issued the first major White House statement about 12:15 a.m. yesterday. He said any penetration of North Korean airspace was "unintentional and regrettable." He also appealed for the return of the crew, requested a prompt explanation by North Korean authorities and called for a meeting of the Military Armistice Commission at Panmunjom.

About an hour later, word came from a North Korean radio broadcast, monitored by the worldwide U.S. network that three of the Americans were dead and one captured after their helicopter "deeply intruded into our area."

After the original brief announcement, which called the U.S. intrusion "illegal" and said an investigation was taking place on the spot. North Korean radio resumed regular programming and said nothing more about the matter during Washington's day.

North Korean officers passed messages through Panmunjom rejecting U.S. requests for a quick meeting and proposing a meeting Saturday morning (tonight, Washington time) when the North Korean investigation is expected to be complete.

The helicopter downing was the subject of comment on Capitol Hill, where some lawmakers were already questioning Carter's plan to withdraw all U.S. ground troops from South Korea within five years. The White House said the incident had caused no change in Carter's plans.

Senate Majority Leader Robert C. Byrd (D-W.Va.) noted a statement by North Korean President Kim II-sung published in Thursday morning newspapers and based on an earlier interview on Japanese television, that he wants to open a dialogue with the United States. "If he is sincere in this, this would be an opportunity for such a dialogue. He could show his sincerity by returning the bodies of the three who were killed and returning the prisoner and the helicopter," Byrd said.

Rep. Lester L. Wolff (D-N.Y.), chairman of the House International Relations subcommittee on Asia, called the incident "further evidence of the intransigence of the North" and said "it has the most serious consequences for us as a nation in our future relations with North Korea."

At the White House, Powell declined to comment on Kim's earlier overture or the effect of the killings on relations with North Korea. Because of concern over the return of the prisoner and the bodies of those killed, "no good purpose" would be served by any statement, he said.

The Defense Department was notifying the families of all four men on the ill-fated helicopter, without being able to tell which of the four is still alive. Names of the crew members were being withheld by the government pending further developments.