Former Argentine president and army chief Alejandro Lanusse, a sharp critic of current military rulers in Buenos Aires has come to their defense in the face of U.S. denunciations of human rights conditions in his country.

Lanusse, 59, was jailed for a month last year at least partly because of his differences with right-wing ruling generals. But the former cavalryman said State Department human rights coordinator Patricia Derian had gone too far with recent charges of continued torture and executions, without any sign of improvement, by the government of President Jorge Videla.

While making clear that much remains to be done, Lanusse said in an interview during a visit here last weekend that there is steady improvement. He gave this account of the consequences of Derian's congressional testimony:

There is a small fascist group in the Argentine army that ordinarily has no importance. But under these circumstances it takes on an undue importance."

Then he asked, "Is it in the U.S. interest to keep throwing wood on the fire, or should it give [Videla] some help?"

Lanusse lamented the "grave error" of the Carter administration in "doing little" to improve ties with Argentina.

Derian's criticisms were extensively reported in Buenos Aires and were taken as part of a concerted campaign against the country. The reports sparked a spate of intense nationalistic feeling in Argentina.

Mark Schneider, deputy of Derian in the State Department rights office, saw Lanusse during his visit, which ended Saturday. Derian was out of the city.

Schneider pointed out that Derian, in her survey or rights conditions for a House subcommittee, had not even mentioned Argentina in the prepared statement. Her assessment came in response to questions.

Videla's government has acknowledged killings and kidnapings by "uncontrolled elements" of the extreme right but has insisted this is ending as the armed forces have virtually eliminated left-wing guerrilla groups.

Many of those now in power blame Lanusse for the conditions of crisis and guerrilla warfare that provoked the military into seizing power two years Lanusse returned the vote to Peronists ostracized by the military for almost 20 years and they easily won general elections. Disruption soon followed.

Criticizing the present leadership for failing to lay out plans for a return to democratic elections, he said, "It cannot delay for very long." But he added that the time is not yet ripe because of what he described as the disaster left by the Peronists.