The United States yesterday strongly denounced South Africa's verdict in the death of Steve Biko, saying the black leader was "a victim of flagrant neglect and official irresponsibility."

In an unusually sharp statement, the State Department said it was "shocked" by the verdict, which cleared police of blame in Biko's death. The department added:

"It seems inconceivable on the evidence presented that the inquest could render a judgment that no one was responsible.

"Even if individual responsibility was not established, Mr. Biko's death clearly resulted from a system which permits such gross mistreatment and violation of the most basic human rights."

Department spokesman Hodding Carter III read the harsh statement in the wake of a ruling by Pretoria magistrate Marthinus J. Prins that the evidence did not show Biko's death "was brought about by any act or omission involving an offense by any person."

Biko, 30, a moderate, died Sept. 12 from head injries sustained while in police custody. He was the 20th detaince to die in police hands in 19 months, and his death caused worldwide protest. Beginning Oct. 19, the South African government has arrested and otherwise restricted more black leaders and their white sympathizers, prompting the United Nations to impose an arms embargo.

The State Department also condemned South African security police for detaining Biko's elder brother, Kaiyo, and his cousin, Solomon Biko, in predawn raids in Johannesburg's black township of Soweto about eight hours before the Prins verdit was announced.

"We naturally find it disturbing that the police would detain other members of the Biko family on the same day as the inquest into Steve Biko's death ends," Carter said. "This action seems particularly insensitive."

Asked if the government is considering any action, such as economic sanctions, against South Africa because of the latest Biko developments. Carter said, "I don't think we have a specific action in mind. We continue to talk to the South Africans about our concerns."

Earlier, Richard M. Moose Jr., assistant secretary of state for African affairs, told reporters. "There is a particular horror about the kind of violence, the kind of treatment, perpetrated on Biko. He was kept naked, handcuffed - to treat a helpless man in that manner and then to ignore very heavy evidence, even if not proven, that there was violence perpetrated against him . . . shows a lock of any apparent concern on the part of the authorities to hold responsible those who had Biko's life in their hands."

Moose was asked about the results of South Africa's Wednesday election, in which the National party of Prime Minister John Vorster won 104 seats in the 165-member Assembly, a gain of 18.

"I don't think it bodes particularly well for the hopes of encouraging a political dialogue between the two races," he said.

Moose said that during the election campaign the South African government seemed to be running against critics outside the country rather than trying "to address issues in a constructive way."

Asked if the size of Vorster's victory might have a moderating effect, Moose said he hoped so. He noted that 30 per cent of English-speaking voters supported the government this time, compared with 15 per cent in the last national election. The increase might mean that more "liberally inclined persons" support the party, and it may have "more of a spread of opinion."

But he added, "That may be clutching at straws."