The Reagan administration yesterday rejected Vietnam's demand for U.S. aid and said Hanoi's action on last year's pledge to help resolve the cases of missing Americans has been disappointing.

The Vietnamese demand and U.S. reaction were evidence of strains that have crept into the two nations' relations despite a mutual understanding reached last August, in which Vietnam agreed to help determine the fates of at least some of the 1,700 U.S. servicemen and civilians listed as missing from the Vietnam war, and the United States promised to encourage private American groups to send assistance to Vietnam, especially to rehabilitate thousands of Vietnamese who lost limbs in the war.

Since then, Vietnam has turned over the remains of eight American servicemen and private medical teams from the United States have proceeded with plans to help in rehabilitation.

The positive public tone of the relationship was buffeted on Tuesday, however, when Foreign Minister Nguyen Co Thach told reporters in Ho Chi Minh City that his country wants direct U.S. aid.

State Department spokesman Charles E. Redman reacted strongly yesterday, saying that "progress to date on the POW-MIA question has been disappointing." He said, "the Vietnamese are fully aware of the firm policy of the United States that {U.S.} aid is out of the question so long as Vietnam continues to occupy Cambodia."