Washington — Human rights under right-wing governments improved in the past year but harsh conditions persisted under communist regimes, the Reagan administration said in a report.
The report, prepared annually by congressional requirement, repeated President Reagan's preference for quiet diplomacy rather than former President Jimmy Carter's public condemnation of abuses to put pressure on repressive regimes. Among the study's findings:
* Latin American and Caribbean countries threatened by insurgencies and those with leftist governments remained the worst human rights violators. In El Salvador, where the civilian-military junta is US-supported, there were signs of improvement last year, although serious human rights problems persisted, the report said.
* In Asia, the pattern of human-rights improvements under right-wing governments, but persistent bad conditions under communism, held true. Vietnam also showed very little improvement and used lethal chemicals supplied by the Soviet Union against dissident tribes in Laos and Kampuchea (Cambodia). In communist China there were signs of a gradual shift ''toward a more open society.''
* For South Africa and its neighbors, racial separation brought increased violence despite proposals for reform.
* The human rights situation in Cuba and the Soviet Union remained bleak. The repression of dissidents in the Soviet Union last year escalated; Moscow used chemical weapons against Asian tribesmen; slave labor was reportedly put to work on the Yamal gas pipeline; and sane dissidents continued to be confined in psychiatric wards.