The Reagan administration turned up its public pressure on the government of South Korean President Chun Doo Hwan yesterday, calling recent concessions to the opposition in Seoul "small signs of flexibility" and bluntly recommending "further concrete positive moves."

The administration's views were announced at a White House news conference by Assistant Secretary of State Gaston J. Sigur, who met reporters immediately after briefing President Reagan on Sigur's just-completed mission to Seoul.

Speaking with presidential authorization against the symbolic background of the presidential seal, Sigur called for Korean opposition leaders as well as the government leaders to "examine their positions and work toward the middle."

Privately, U.S. officials are showing signs of impatience at the unwillingness of Korean opposition leaders to renew negotiations with the Chun regime on the means of selecting Chun's successor between now and February, when the president is pledged to step down.

Sigur had no word of criticism of the opposition in his remarks, though he did say that the Korean people "ask for consensus to replace confrontation, meetings to replace posturing and for compromise to pave the way to progress."

The most explicit message was for Chun. "We urge tolerance for peaceful demonstrations, release of people in prison for political activities, an end to preemptive arrests and house arrests. Such steps are necessary," Sigur said.

By describing as "small signs of flexibility" Chun's meeting with opposition leader Kim Young Sam and Chun's reversal of his April 13 decision ending national debate on constitutional change, Sigur appeared to be suggesting that the Korean president should go much farther toward meeting opposition demands.

"This is a very tense time in Korea, full of opportunities for progress toward political compromise, yet carrying risks of degeneration into violent confrontation," Sigur said. As in other recent official U.S. utterances, there was no mention of North Korea or the international security situation.

Sigur made it clear that Washington opposes renewed house arrest for opposition leader Kim Dae Jung, though he suggested it is understandable in the context of yesterday in Seoul, when mass demonstrations had been threatened to test the strength of the government and opposition. The U.S. official said he was "very hopeful" that Kim's renewed house arrest will be lifted "very shortly."

The U.S. message to the Korean government and military commanders, as expressed by Sigur, continued to emphasize that "military steps offer no solution" to the political crisis. Sigur said it is "clear as day" to everyone that in the U.S. view, "any intervention by the military into this situation is unwarranted and we oppose it."

U.S. officials, who expressed relief that a day of tension and testing had ended in Seoul without the explosive outbreak of disorder that had been feared, said Sigur's public message was designed to affect Chun's decision making as he ponders new steps to assuage the public and political opposition there. Announcement of further government steps is expected from Chun within the next several days, officials said.

U.S. apprehension about the political situation in Korea and U.S. policy toward it, which have grown clearer and sharper over the past two weeks of large-scale public demonstrations, escalated further with Sigur's news conference. Sigur previously said he had gone to Korea to listen and assess the situation rather than prescribe. But after reporting to Reagan on his mission, Sigur insisted that major new democratizing steps be taken by the Chun government, came closer than before in specifying what they should be, and did so in a more highly visible and symbolic public setting.

Sigur disavowed any intention by Washington to "intervene in detail" about the political process in Korea. Nevertheless, he said, the United States must continue to strongly urge that the transfer of power from the retiring Chun take place as scheduled next February and that it take place "in peaceful circumstances . . . with continuing and growing dialogue between all parties concerned."