South Korean generals broke a longstanding agreement with American forces when they withdrew front-line troops to assist in their internal power struggle last week, reliable souces said today.

A South Korean Army battalion, positioned with other forces to guard against a North Korean attack close to the Demilitarized Zone, was pulled back into Seoul about midnight Wednesday to secure control of the capital for insurgent generals, the sources said.

That violated a key agreement, long observed by the U.S. and South Korean commanders, that neither country's forces can be moved without approval of the other.

The unauthorized movement is believed to have deeply angered the American command. A spokesman for the U.S. 8th Army said it was regarded as "a very serious matter."

He said the commander of U.S. forces, Gen. John Wickham, had "talked it over" with the then South Korean defense minister, Gen. Ro Jae Hyun, who also had tried to stop the movement. He denied widespread reports here that Wickham had lodged a protest with the insurgent generals who ordered it.

The spokesman said the movement was at least "a violation of courtesy and professional ethics" but said it is not clear whether it violated a formal written agreement between the two countries. That point is being studied, he said.

The affair is expected to cause friction between the U.S. and South Korean forces. Any troop movements are supposed to be authorized by a combined forces command, which Wickham also heads.

Even in the tense days preceding and following President Park Chung Hee's assassination on Oct. 26, South Korean security movements were cleared with the joint command. Local observers said an unauthorized movement had not occurred since 1961, when Park led a military coup that toppled a civilian government.

This time, sources said, the American side was informed only after the fact.

The incident occurred about five hours after a group of generals arrested Army Chief of Staff Gen. Chung Sung Wha, who had been martial law commander since the assassination.

Chung's military allies resisted and after his arrest, about 7 p.m. Wednesday night, engaged in a gun battle at his official residence. One general was killed, it was confirmed today.

By midnight, Chung was in custody, but the insurgents did not have control of Seoul's key government buildings, which had been guarded by a capital garrison force whose commander was loyal to Chung.

Shortly after midnight, the sources said, the insurgents ordered a battalion from the 9th Division to leave its position north of Seoul and take up posts in the capital. The troops already in the capital backed down without a fight. The battalion included an armored company.

Tanks from the armored brigade remained in the capital at least 24 hours and were seen rolling through downtown Seoul in the early morning hours Thursday, apparently returning to their battle-front position.

During much of the confusion and two gunfights Wednesday night and Thursday morning, U.S. Ambassador William Gleysteen and Gen. Wickham were in a headquarters bunker of the combined forces command located within the U.S. 8th Army compound.

With them was the then-defense minister Gen. Ro, who reportedly tried at first to make the insurgent generals back down. He failed and apparently was forced to make the first formal annoucement that Chung had been arrested.

Meanwhile, speculation continued today about the nature of the intramilitary coup and about the political course to be taken by the newly powerful generals who staged it.

They have said nothing publicly about their motives and goals. Their spokesman has said only that they acted to arrest Gen. Chung because he was suspected of complicity in the plot to assassinate Park.

A number of diplomats and other sources here say that if that is true it is only part of the reason. They believe there were several motives. One of them was a deep animosity toward older generals who, they believe, have been corrupted and enriched by many years in command and who became even more deeply entrenched when Chung rose to great national power as martial law commander.

The insurgents generally are younger than the military generation represented by Defense Minister Ro, who was one of their chief targets and who has been replaced.

Sources pointed out that several of them are from two graduating classes of the Korea military academy. One of those classes, the eleventh, was the first to graduate from a four-year program of military training, and they are said to look down upon older generals who had scant formal training and no university degrees. One source said today, "They tend to look on themselves as the first real professionals in the South Korean military."

Some sources also think that a common bond was a strong sense of nationalism that made them less pleased than their elders to accept the American "big brother" role in South Korea's military and civilian affairs.

The reportedly felt close to the late president Park because of his repeated insistence on South Korean "self-reliance," which had become a kind of code word for a latent anti-Americanism, even though they willingly accepted the American role of military protector.

One diplomat said it was this nationalist sympathy that bound them in loyalty to Park, rather than approval of the president's harsh domestic rule. He cautioned it is not correct to describe them as "hawks" who want to turn back the clock to Park's authoritarian constitution.

Most sources here believe that the takeover was led by four generals, the most prominent of whom is Gen. Chon Doo Whan. He was the commander of Army counterintelligence and was in charge of the joint investigative command which, under Gen. Chung, was assigned the task of investigating the assassination. His command's official report cleared Chung of any blame in the plot, but last Wednesday Chon led the forces that arrested him.

Another of the four is Gen. Ro Tae Woo, commander of the 9th Division whose armored brigade was shifted to Seoul on the night of the takeover.

In the days following that seizure, the role of Gen. Lee Hui Sung has become more murky. A protege of Chung, he was named Thursday as Army chief of staff and the new commander under martial law, a position of vast power. Sources now say he played no part in the takeover but has since joined the insurgent generals. Some reports depict him as serving at the whim of Gen. Chon and the other three principals in the plot.