An Anglo-American proposal for the appointment of a special U.N. envoy to help negotiate a cease-fire in Rhodesia won wide approval in the Security Council today, but Soviet delaying tactics postponed final action until Thursday at the earliest.

For the first time Joshua Nkomo, a leader of the Patriotic Front gave the British initiative his approval. The Front is the main black nationalist group carrying on the guerrilla war against the white-minority government.

All the council members, except for China, the Soviet Union and Libya apprived the proposal and both Western and African diplomats expressed confidence that the resolution would be adopted in the end.

The Soviet Union wanted to blunt the Anglo-American peace effort completely by preventing the United Nations from taking the first step of naming a representative according to the plan's backers.

But since the resolution had the support of the so-called "front line" African states, Moscow coule not afford to oppose it openly.

The significance of U.N. support for the cease-fire negotiations is that it keeps the broader peace plan alive. Equally important, according to U.S. Ambassador Andrew Young, it puts the negotiating process into the hands of the military leaders on both sides - the Rhodesian armed forces and the Patriotic Front.

Young, who accompanied British Foreign Secretary David Owen on a recent tour of Africa to promote the peace initiative, said the military leaders were the officials most receptive to negotiation.

Under the terms of the British resolution, Secretary General Kurt Waldheim would appoint a representative "to enter into discussions, with the British resident commissioner-designate [retired Field Marshall Lord Carver] and with all the parties, concerning the military and associated arrangements that are necessary to affect the transition to majority rule in Southern Rhodesia."

Owen in introducisng his resolution today, described it as a limited step toward a settlement that would not commit council members to other aspects of the wider British-American peace plan for Rhodesia.

Joshua Nkomo said that if the purpose of the U.N. presence is facilitate the decolonization of Rhodesia, "the Patriotic Front will have nothing against it."

But he asked that the representative's duties be "more specifically defined" in close consultation with the Security Council and the parties to the armed conflict.

Weatern and African diplomats said Nkomo set this condition under Soviet pressure. It conforms to Moscow's long-held stand that the Security Council, and not the secretary general must control any U.N. peace-keeping ventures.