JOHANNESBURG, South Africa (AP) _ Two extreme right-wing groups signed a cooperation agreement Sunday in the latest effort by pro-apartheid whites to build opposition to reforms instituted by President F.W. de Klerk.

The groups also pledged support for a white fugitive who has declared war on the government.

Both de Klerk and African National Congress leader Nelson Mandela have warned that heavily armed right-wing extremists could undermine efforts to launch black-white negotiations on a new constitution and ending apartheid.

The Boer Resistance Movement and the Boer State Party announced Sunday they would work together to establish a homeland for the Boers, or Afrikaners, the country's Dutch-descended settlers who account for 3 million of the 5 million whites.

The two groups, which are believed to have several thousand followers among them, are among several right-wing organizations that favor strict segregation and oppose all concessions to South Africa's 28 million blacks.

The organizations consider the ANC, the country's largest black opposition group, a communist-led terrorist movement.

The right-wing groups ''promised all possible legal support'' for Piet Rudolph, a member of the Boer State Party who declared war on de Klerk's government last week.

Rudolph has been in hiding since he allegedly stole arms and ammunition from an air force base in April. He made his declaration of war in a videotape sent to a newspaper. The government has offered a $19,000 reward for information leading to Rudolph's arrest.

The two Afrikaner groups said they had established a fund to help ''all the other unknown Piet Rudolphs ... who have to flee from the de Klerk-Mandela Communists.''

Right-wingers have been threatening the government for months. Events of the past week provided the strongest indications yet that such groups are ready to act.

Among them:

-Two offices belonging to de Klerk's National Party were bombed Friday night, causing extensive damage but no injuries. No one has claimed responsibility, but government officials say right-wing whites are the most likely suspects.

-An Afrikaans-language newspaper reported Friday that white extremists were planning to assassinate Mandela and de Klerk and poison drinking water in Soweto, the huge black township outside Johannesburg. Police arrested 11 whites for questioning, but all were released Friday and no charges have been filed.

Most political analysts believe that an increasing number of whites are joining right-wing groups in response to de Klerk's reforms. The pro-apartheid Conservative Party captured 31 percent of the vote in white elections last September and would probably improve on that figure if polls were held at present, analysts say.

De Klerk says he wants to end racial discrimination in South Africa and negotiate a constitution that allows blacks and whites to share power. Under the current apartheid system, whites control the government and the economy.