Private groups that monitor U.S. human rights policy abroad have said they are detecting signs of modest improvement in Reagan administration words and deeds, but the State Department's human rights chief says the only changes are in "tactics."

Assistant Secretary of State Elliott Abrams made the comment in an interview after two unusual administration steps last week in opposition to political repression in Chile and South Africa.

Last Thursday, the U.S. representative on the World Bank board abstained from a vote approving an $11 million loan to Chile. The action, which followed similar U.S. abstention Feb. 7 on a $130 million loan by the Inter-American Development Bank, was described by officials as symbolizing U.S. dissatisfaction with Chile's crackdown on human rights and political liberties.

Last week, the United States also cast a rare vote against South Africa in the U.N. Security Council, supporting an African bloc resolution that condemned the killing of demonstrators in the Crossroads shantytown near Cape Town and the arrest of black political opponents on allegations of treason. The administration usually has vetoed resolutions attacking South Africa's policies rather than abstaining on or voting for them.

"There has been no change in policy. Everything we are doing now has been part of our policy," Abrams said in the interview.

"Tactics change whenever they need to," he added. "Quiet diplomacy is a tactic and not a matter of principle."

Aryah Neier, vice chairman of Americas Watch and Helsinki Watch, said he has detected "some shift" in policy with regard to countries where major U.S. geopolitical interests are not seen to be at stake.

"Abrams has a consistently good record on Chile and probably also in respect to South Africa," Neier said. "On the other hand, his role in El Salvador has been as an apologist who misrepresented the situation."

In their annual report on U.S. human rights performance in 1983, Americas Watch, Helsinki Watch and the Lawyers Committee for International Human Rights wrote that the administration's policy in that year "was a failure . . . . Except in a handful of cases it failed to promote human rights."

Two months ago, the same groups said in their next annual report, "Though there were a few bright spots in the Reagan administration's performance on human rights during 1984, overall its record was dismal."

Patricia Derian, State Department human rights chief in the Carter administration, said "a little lip service" to human rights has developed in recent Reagan administration policy. She added that "the change doesn't mean anything" except that "somebody's awake over there."

Concerning Chile, Abrams said, "the change came in Santiago, not in Washington." He said that a period of "real political opening" was ended last summer by the government of President Augusto Pinochet, and that the United States began to react last September with public statements and a series of symbolic moves.

In the latter category, he cited cancellation of a trip to Santiago by Army Chief of Staff Gen. John A. Wickham Jr. last November and the bank loan abstentions this month and last.

The abstentions, Abrams said, "send a signal to Santiago without casting our position in concrete." To vote against the loans rather than abstain, he said, "might send a stronger signal but would tend to restrict further flexibility on our part."

Asked if the administration will continue to abstain on loans to Chile and show other signs of open disapproval, Abrams replied, "It depends on what will work, on what policies the government follow, what the overall political situation is, how the democratic opposition is acting, how much unity there is in the opposition and how much terrorism there is in Chile."

Regarding South Africa, he maintained that "it's not the policy that has changed but the effort to explain it at home."

Abrams said U.S. policy under President Reagan has been paying off in "extraordinary changes" in South Africa.

Nonetheless, he said, "we have had to change our public strategy at home, because it was clearly not getting through to a lot of people." Abrams was referring to growing demonstrations here, in which more than 1,500 persons have been arrested for protesting outside the South African Embassy.

Abrams minimized the significance of last week's Security Council vote, saying the administration always has been willing to vote against apartheid if the language at hand was acceptable.

Last week's resolution to "strongly condemn" policies of "racist South Africa" was simply the first in the Security Council in a long time that the United States could accept, Abrams said.