JOHANNESBURG, JULY 23 -- South African President Frederik W. de Klerk warned the African National Congress tonight that he views "in an extremely serious light" evidence that the group's military wing has been preparing for armed action and said its military commanders could be arrested.

De Klerk's warning followed the roundup of at least 40 African nationalists who allegedly belong to the ANC's military arm, Spear of the Nation. Police have alleged that the organization has been infiltrating men from neighboring states and stashing supplies of arms in preparation for an "insurrection" should negotiations for a political settlement with the government fail.

De Klerk said the police investigation into the suspected plot would continue and that if evidence pointed to criminal action, "the accused will be heard from in open court."

It was not immediately clear why de Klerk had waited all day before issuing a statement concerning the police crackdown on African nationalists. But the arrests of top ANC military commanders would almost certainly halt the negotiations.

Referring to the three-month immunity granted to the ANC's top leaders, de Klerk reminded them that the measure only applied to acts committed before it came into effect in mid-May and had been granted to those who "would otherwise have faced prosecution."

While de Klerk did not mention any names, it was clear that the warning was aimed at the ANC's military leaders, notably chief of staff Chris Hani. He had threatened last week to "seize power" if negotiations with the government faltered and said he was continuing to recruit and strengthen the Spear of the Nation forces.

"The peace and immunity processes do not provide room for anybody to sneak in through the back door and try to seize power in the country by force," de Klerk said.

De Klerk added that there were "certain basic rules" inherent in the process of peaceful negotiations and that these did not allow "for any political organization to stockpile arms in order to force its will at the negotiating table."

Despite the sudden confrontation between the government and the ANC over the arrests, both sides have indicated that they intend to go ahead with their next round of talks scheduled for Aug. 6 and hope to remove the remaining obstacles to full-scale constitutional negotiations.

The U.S. Consulate in Durban became involved in the police roundup of ANC activists today as one of them sought refuge there and asked for political asylum.

The ANC office in Durban identified the man as Bhekumusa Jabulani Ximba, 38, a senior laboratory technician at the University of Zululand, and described him as an ANC activist. The South African Press Association said Ximba was believed to be a member of Spear of the Nation.

A U.S. Embassy spokesman in Pretoria said that the U.S. government was in the process of trying to confirm information on Ximba and the nature of his "dispute" with South African authorities.

Another U.S. official said that the embassy was in contact with the South African government, but it was not clear whether Ximba was seeking asylum or just protection from the United States. Ximba was still on the consulate premises after the close of office hours.

Also today, the Communist Party's general secretary, Joe Slovo, issued a statement denying accusations that his party was involved in "some sinister red plot" and said that the government was indulging in "red-scare tactics." Many of those arrested in the police crackdown also have claimed to be members of the Communist Party, according to local press reports.

Slovo said the recent arrests were an attempt by de Klerk to disrupt the Communist Party's plan to hold its first public rally next Sunday in Soweto. "His attempt will fail as surely as every other endeavor to eradicate Communist ideas and Communist organization from the soil of South Africa," Slovo said.

Reacting to press reports that the Communist Party might be acting outside the ANC leadership in preparing an insurrection, Slovo said that party members who belonged to the Spear of the Nation "do not fall under any separate {Communist} party command."

Meanwhile, the leader of the white right-wing Boer State Party, Robert van Tonder, issued a statement demanding that 31 convicted or suspected white terrorists be released immediately. This is a far greater number of right-wing extremists being held by police than had previously been reported.

Van Tonder said that all but four were being held under Article 29 of the Internal Security Act, allowing police to hold the right-wing party members indefinitely and without access to a lawyer or family members.