Britain and the United States reported a positive start here yesterday for a conference culiminating their five month effort to get the Rhodesian black nationalist Patriotic Front to seriously consider an Anglo-American plan for a peaceful settlement of the Rhodesian conflict.

The British convened talks adjourned earlier than expected in mid-afternoon to allow each party to prepare more detailed proposals for today's session.

British Foreign Secretary David Owen and U.S. Ambassador to the U.N. Andrew Young expressed surprise and pleasure at the front's position. The talks had been expected to bog down because of the front's opposition to many provisions of the Anglo American plan.

Young described the atmosphere during the two-hour morning session and an hour-long one in the afternoon as "very good."

"There was a serious deliniation of the questions we are divided on and a decision that we ought to work the next day or two to see if we cannot work out our differences," he told reporters.

Owen said there was "no bitterness and no rancor." It could be, he added, the augury for eventually coming together in compromise and accommodation that I believe will be necessary" for a settlement.

Joshua Nkomo, co-leader of the Patriotic Front, remarked that there was "a seriousness on both sides to move forward" in a search for an internationally acceptable solution to the 13-year-old dispute.

British and American spokesmen said that the rival negotiations presently being held by Rhodesian Prime Minister Ian Smith and three other black nationalist leaders based in Salisbury were only "casually" mentioned during the opening round of talks.

The sudden prospect of an accord being reached in Salisbury on the formation of a multiracial interim government as an alternative to the British-American settlement plan appears to be one major factor in the Patriotic Front's sudden willingness to enter into detailed negotiations with the two Western powers.

Another important factor, according to American and British conference sources, has been the behind-the-scenes role of Mozambique's president, Samora Machel. He is reported to have become a strong supporter of the Anglo-American approach and to have pressed the front recently to enter talks on it.

The Western plan calls for Smith to surrender power to Britain, the former colonial power in Rhodesia prior to the unilateral declaration of independence by the whites there in 1965. Britain would then organize elections within six months on the basis of adult universal suffrage that would lead to a new black majority government.

The rival settlement plan being negotiated in Salisbury would give no role to the British in the interim government and would probably shut out the Patriotic Front from any elections.

Young reacted sharply to a suggestion that the Salisbury talks might upstage the Malta conference.

"Why? How?" he asked. "Nothing in Salisbury can stop the fighting. There is a slight possibility that we might take some steps in that direction here."

Conference sources said the issues discussed yesterday included a ceasefire in the worsening guerrilla war and arrangement for the proposed British-run interim government leading up to elections. The front opposes the all-powerful British role in the transition period and wants Britain to hand over power directly to it.

Owen's remarks seem to confirm reports that the British and American negotiators are seeking a formula to include front representatives in the transitional government without giving up British control over it.

The talks are being held in the rooftop lounge of the Grand Verdala Hotel overlooking this picturesque mediterranean isle. Rabat is a fortress-like town built by the Arabs.

Participating in the three-day conference in addition to the British and American delegations are the two co-leaders of the Patriotic Front, Nkomo and Robert Mugabe; the British resident commissioner-designate for Rhodesia, Field Marshal Lord Carver, and the special U.N. representative for the Rhodesia issue, Gen. Prem Chand.

The black leaders involved in the talks going on in Salisbury are Bishop Abel Muzorewa, the Rev. Ndabaningi Sithole, and chief Jeremiah Chirau. Muzorewa walked out of the talks Friday after he accused Smith of insulting him. He said he would not return until the Rhodesian leader apologizes.

It is strongly suspected here that this was a ploy by the bishop to play for time to gauge the results of the Malta talks before any further negotiations with Smith.