Prime Minister Ian Smith said yesterday that the Rhodesan constitutional agreement promising black majority rule by the end of the year will not be carried out unless there is a cease-fire in the six-year-old guerilla war.

Yet he did not rule out the possibility of the biracial government joining peace talks with two guerrilla leaders as proposed by the United States and Britain.

Smith, speaking at a news conference, asserted that the British-American plan as it stands guarantees a handover to the guerrillas. He said he and three black colleagues heading the transition government would consider attending all-party talks on "more realistic terms."

Smith chided his black partners in the four-month transition government for what he termed the "lack of progress toward ending the fighting."

"Don't let's be mealy-mouthed about it. This is clearly a field in which my black colleagues operate almost exclusively and I hope we've going to have better results from now on," Smith said.

In apparent criticism of the three black leaders who sit with him as leaders of the government, Smith said. "I'm disappointed . . . I have made my view known to my colleagues on the executive council on one or two occasions."

Smith insisted the entire constitutional accord guaranteeing majority rule hinges on a cease-fire.

The 80,000 mainly white voters are expected to vote in a referendum in September or October on a proposed black rule constitution. If endorsed as most observers believe it will be, the country's first multiracial election will be held.

Yet Smith told about 60 foreign and local jornalists: "If we don't succeed in implementing the agreement, then I fail to see how we can put this to the (white) electorate." Whites wielded political power in Rhodesia for 90 years until the recent settlement laid the groundwork for a black-majority government next year.

Smith refused to say as what point he might consider the settlement to have failed.

Leaders of the Patriotic Front guerrilla alliance rejected the Biracial agreement and pledged to disrupt proposed elections.

Two black transition government members, Bishop Abel Muzorewa and the Rev. Ndabaningi Sithole, have made repeated appeals to guerrillas to stop fighting - and both maintain their efforts are having some success.

(Bishop Muzorewa, who is visiting Washington, yesterday denied that Rhodesia had failed to deal with intensified guerrilla warfare. While refusing to comment directly on Smith's remarks without seeing the text, he said the situation should not be judged by the recent flurry of attacks on missionary posts. He said the missions are soft targets and attacks on them do not indicate the true state of affairs.

(Muzorewa is in Washington to support moves to have Congress lift trade sanctions against his country.)