Rhodesia's interim multiracial government announced yesterday that it opposes the 'all-parties conference' championed by President Carter to avert civil war between rival nationalist leaders. But left open the possibility of further talks.

The announcement further darkened the prospects for this week's projected trip to Africa by Secretary of State Cyrus Vance to bring Rhodesia's feuding factions into peace talks.

"We don't take this as a definitive position" a senior State Department official said last night in commenting on the Rhodesian announcement. Vance, he said, remains "determined to to the extra mile to try and resolve this situation."

Vance told the American Society of Newspaper Editors at luncheon Yesterday that he has no "assurances" that the internal nationalist faction that joined the government in Rhodesia can be brought together with the guerrilla forces based outside the country that are fighting for black majority rule.But he said it is of "tremendous importance" to avert civil war and to achieve a Rhodesian peace settlement.

Along with British Foreign Secretary David Owen, Vance plans to meet in Tanzania April 15-16 with Joshua Nkomo and Robert Mugabe, leaders of the Patriotic Front, the militant Rhodesian black nationalist organization. Vance yesterday moved up his departure a day, to leave Washington on Wednesday, for Dar es Salaam, Tanzania, where that meeting now has been shifted. The talks are also to include leaders to the five "front line" nations neighboring Rhodesia.

From Tanzania, Vance and Owen intended to go to briefly to Salisbury, the Rhodesian capital. There they are planning to confer with Prime Minister Ian Smith and the three moderate black leaders who have joined Smith their own transitional government - Bishop Abel Muzorewa, the Rev. Ndabaningi Sithole and Chief Jeremiah Chirau.

That intermim government yesterday spurned the proposed talks with the Patriotic Front after four hours of talks with an Anglo-American diplomatic team.

In a statement, the New Rhodesian executive council said it concluded that its "most important and urgent task" is to proceed with its own plans to install a government of black majority rule in Rhodesia by Dec. 31.

As a result, the council said, it is "therefore not in favor or reopening negotiations through the proposed all-parties conference."

If Vance and Owen "would like to have detailed discussions with the executive council," it said, "they will be welcome to come to Salisbury next week as proposed."

Carter administration officials interpreted that as "an ambivalent position," and one "not unlike the line they have been taking," although it hardened the public posture.

The interim government made the additional point yesterday of invoking protocol as heads of state, leaving it to deputies to confer with the Western envoys, John Grham of the British Foreign Office and Stephen Low, U.S. ambassador to Zambia.

U.S. officials said they will await further reports from the two envoys, and Washington-London consultations, before deciding whether Vance and Own will go to Salisbury for talks originally projected for April 17.

President Carter in Lagaos, Nigeria, early this month announced plans for the intended "all-parties conference' to try to bridge differences between a proposed Anglo-American plan and the Rhodesian internal settlement for the nation of 6.7 million blacks and 260,000 whites.

Prime Minister Smith, in an interview last week, however, continued to refer to Patriotic Front leaders Nkomo and Mugabe as "terrorsts" who have excluded themselves from Rhodesia's future until they "renounce violence." he also said that "the Anglo-American proposals (for Rhodesia's future) are considered defunct" by all members of the executive council.