The United States has responded to international unrest in South Korea with new political signals of concern and disapproval, including a letter from President Carter and an unannounced change in procedures for considering economic development loans to that country.

At the same time, new expressions of support for Korea's security by Defense Secretary Harold Brown have complicated the human rights diplomacy.

The mixed result of a high-level Washington policy review, which came to a head late last week, took into account a complex and sometimes conflicting welter of U. S. political, economic and security interests in Korea. In view of the problems involved, policymakers have been cautious both in their diplomatic decisions and in public disclosure of them.

A senior State Department official stressed that the U. S. objective is not to bring down the troubled regime of President Park Chung Hee but to convince Park to emphasize conciliation, rather than confrontation, with the political opposition. Washington's hope is that such a shift would restore a measure of stability to South Korea, where martial law has been declared after riots in the second largest city, Pusan.

The most dramatic public sign of Washington's displeasure was the recall two weeks ago of Ambassador William Gleysteen for consultations. This was announced the day after the expulsion of Korean opposition leader Kim Young Sam from the National Assembly, an event that deepened already intense political discord in Seoul.

Results of the Washington consultations included:

The letter from Carter to Park expressing concern about the recent events and making clear, according to officials, that the future course of relations between Washington and Seoul is at stake in Park's current decisions. Officials reported that the letter did not outline a specific course of action that the United States would like Park to take.

An unannounced meeting at the State Department last Saturday at which Secretary of State Cyrus R. Vance expressed strong U.S. concern to Korean Ambassador Kim Young Shik for transmission to Seoul.

A decision, conveyed to Seoul, to suspend routine U.S. approval of economic development loans for Korea proposed by the Asian Development Bank and other international financial institutions.

Washington abstained on such loans because of human rights considerations for several months late in 1977, but hasconsistently voted in favor of loans to Korea, with greater likelihood that Washington will abstain or vote against them.

A $25 million coal development loan scheduled to be considered by the Asian Development Bank next week may be the first test of the new U.S. policy. Some officials suggested that Washington may ask that consideration be postponed for a more intensive review.

Running counter to the signals of concern is the message of reassurance projected by the current visit to Seoul of Defense Secretary Brown. The Brown mission for periodic U.S.-Koran military consultations had been scheduled before the internal discord in Seoul reached its new intensity.

There is no indictation that serious consideration was given to postponing Brown's trip or replacing him with a lesser official. The Carter administration is concerned about any action in the security field that could transmit a signal of weakening resolve to North Korea, especially after widespread charges that this was the effect of Carter's plans for withdrawal of U.S. ground troops.

Another problem for American policy is the possibility that public expressions of U.S. displeasure could spur Park's political opponents to stronger action while failing to convince Park to take a moderate course. The Carter administration is hopeful -- but by no means confident -- that the steps to date will succeed, making more difficult and more visible steps unnecessary.