:U.S. Secretary of Defense Harold Brown said today that the American security commitment to South Korea will not be used to exert pressure for political change in the field of human rights.

Brown told a news conference he believes that any attempt to pressure the Seoul government could endanger the national security interests of both countries.

"I do not believe that any attempt by the United States to manage the U.S. security role here to achieve some particular political objectives would benefit either our long-term strategic interests or contribute constructively to political developments in Korea," Brown said.

Some opposition forces here had hoped that the United States would use its defense commitment to South Korea as leverage to bring about a more liberal human rights policy by the government of President Park Chung Hee. Brown's comment, which was volunteered at the news conference, was the clearest public statement so far that the defense role would not be used in that way.

Brown also said he doubts that even Park's opponents would want any lessening of a security commitment by the United States.

"I doubt that any significant political grouping in Korea would advocate even a symbolic diminution of the security cooperation between the United States and the Republic of Korea," Brown added.

Brown wound up two days of defense review here this morning by promising a modest strengthening of American armaments while steering a cautious course around the delicate issue of this country's current internal political troubles.

Officials in Brown's party disclosed a step-up in artillery, helicopter, and some aircraft forces. They also opened the door for South Korea's first domestic production of fixed-wing military aircraft.

No big surprises were revealed in the military package and some other key South Korean arms requests were deferred or denied.

U.S. sources said after today's meetings that Brown and associates had hoped to prevent the country's internal unrest and political tension from becoming involved in defense issues.

[WORDS ILLEGIBLE] President Park imposed martial law on the city of Pusan after two days of student rioting and promised to root out the "impure elements" that had participated in it.

The riots in part were a protest of the government party's expulsion from the National Assembly of the opposition party leader, Kim Young Sam, two weeks ago.

Brown met today with Park and, according to U.S. officials, save him a letter from President Carter, which it is believed touched on the government party's expulsion of Kim. The expulsion issue caused the temporary recall of American Ambassador William Gleysteen as a sign of Carter administration disapproval.

There had been advance reports that the Carter letter would contain a measured rebuke of the expulsion. U.S. officials tonight gave a guarded description of the letter, implyng that it was somewhat critical of the Park government on political matters but made no demands for a change of course.

They said Carter's letter reviewed some developments here since his state visit in June. His comments were "positive" on the South Korean record on security matters but "less postive" on political developments, the officials said. They said Carter made no requests of Park in the political field.

The officials were equally guarded in discussing the 1 1/2-hour conversation between Brown and President

Park, saying only that it followed the lines of Carter's letter.

Park's imposition of martial law in Pusan early today was not discussed, the officials said, asserting there was an absence of information on the American side.

The American officials made it clear that they want to keep defense issues insulated from political pressures here and in the United States.Brown's purpose in coming was to attend an annual defense review.

They acknowledged, however, that they are concerned about public opinion in the United States insofar as it is affected by Park's politics.

Meanwhile, Pusan was reported quiet tonight as armed forces patrolled the streets protecting government buildings and other key facilities in that port city 200 miles south of Seoul.

On Tuesday and Wednesday nights about 3,000 students from two universities poured into the streets shouting antigovernment slogans, some of them in protest of Kim's expulsion. They demolished police cars and substations.

More than 200 were arrested and about 50 police injured, the government said. Both universities were closed in the martial law order and unauthorized meetings were banned.

President Park denounced the demonstrations as "threatening the foundation of constitutional rule" and said it was caused by "students and impure elements disregarding the interests of the nation and the welfare of the people."

In the defense meetings here today, American officials gave notice that they intend to upgrade U.S. artillery strength, provide more helicopters, and deploy a new squadron of aircraft used for close ground support in the event of a North Korean attack.

The major U.S. offer, from the South Korean viewpoint, was Carter administration approval of an arrangement to let the South Koreans assemble their own F5 jet fighters. Major parts would be shipped here by the U.S. manufacturer, Northrup, and produced by South Korean labor under American technical supervision.