Senior administration officials yesterday defended their policy toward southern Africa, a policy that is coming under increasing attack as a result of Washington's refusal to condemn South Africa unilaterally for its invasion of Angola.

The Reagan administration risks becoming isolated not only from black African nations but from its European allies if its policy of working more closely with South Africa to find a Namibian solution fails to bring results.

Senior officials called reporters to the State Department yesterday to answer questions about the southern Africa policy, which was formally outlined in a speech by Chester A. Crocker, assistant secretary of state for African affairs, last weekend.

The officials gave no concrete examples, but said the administration has established "a useful dialogue" with South Africa. "We think progress has been made."

In effect, the officials asked that judgment of the policy be withheld until its results are known. They said they share the urgency felt by black African and some European nations about seeing Namibia receive its independence from South Africa.

They said it was a "strange irony" that all the African nations tell the administration they recognize the efforts of the United States and its four partners in the group trying to find a peaceful Namibian solution (Canada, Britain, France and West Germany) are essential while at the same time they introduce public debates at the United Nations that threaten splits in the group of five.

The United States on Monday vetoed a U.N. Security Council resolution condemning South African. Britain abstained in the vote; France voted for the resolution. West Germany and Canada are not currently on the Security Council.

"We all share the same objectives," the senior U.S. officials said of the group, denying that the U.N. vote will have any lasting effect on the group's efforts.

Soon, the administration will present to all the parties involved-- South Africa; the rival factions in Namibia; SWAPO (the South-West Africa People's Organization), which is recognized by black Africa as the sole representative of the Namibian people; black southern African nations, and the European allies -- a number of suggestions, which the senior officials stressed are designed to strengthen the U.N. resolution calling for Namibia's independence.

Secretary of State Alexander M. Haig Jr. "made abundantly clear" to a delegation of Organization for African Unity ministers Monday the United States' dedication to early implementation of the U.N. resolution, the officials said. Any U.S. suggestions will not amend the U.N. resolution, but strengthen it, they added. "It is intact," they said of the call for South Africa to release its hold over the last sizable territory in Africa still under colonial rule.

They indicated sympathy, however, for South Africa's political problems in granting independence to Namibia. They called it a fundamental decision of "real gravity in South African politics."

The administration thought it was important that the South African invasion be viewed in a wide context, the officials said. Therefore, the United States deplored the invasion, but equally deplored other acts of violence in the area, and stressed a link between the invasion and the presence of 15,000 to 19,000 Cuban troops in Angola along with about 1,000 Soviet and East German advisers and large amounts of Soviet-supplied equipment.

The next steps in the Namibian process will come Thursday when the U.N. General Assembly holds a special session on the question. The session is certain to feature denunciations of South Africa by black African and Soviet-bloc states.

The foreign ministers of the five-nation group seeking a peaceful solution plan to meet Sept. 24.