President Carter yesterday continued U.S. economic sanctions against Zimbabwe-Rhodesia, saying that lifting them at this "critical stage" of the Rhodesia peace conference in London could jeopardize its outcome.

But he said the sanctions, imposed after Rhodesia's white minority unilaterally declared independence from Britin in 1965, may be lifted if the conference between warring black Rhodesian groups fails to reach agreement on genuine majority rule.

The president said that "encouraging progress" has been made at the British-sponsored talks between Prime Ministe Abel Muzorewa, who took power in Salisbury with the support of white Rhodesians, and Patriotic Front guerrilla leaders opposing Muzorewa.

"We hope they will be rapidly and successfully concluded," Carter said of the talks.

A senior State Department official said an accord on transition to black majority rule and "impartial elections" may come "within days, perhaps 10 days."

The president was required by law to end the sanctions by today unless he determined that doing so was not in the national interest. Congress can

A leading opponent of sanctions, SEN. Jesse Helms (R-N.C.), said last night that he will ask Congress to overrule Carter.

Senior State Department officials said they will urge Helms to await the outcome of the London talks before proceeding.

"The talks are very close to an outcome," one official said. "We hope the senator would keep that in mind."

Meantime, Britian permitted some of its economic sanctions against rhodesia to lapse today while continuing its ban on direct trade with Rhodesia and on financial transfers between itself and Rhodesia, the two most important elements in its sanctions policy.

A U.S. official said there was no practical way of lifting selected U.S. sanctions against the Salisbury government "since our restrictions have been lumped together" in an amendment attached to the State Department spending authorization.

Before announcing his decision, Carter and Vice President Mondale met with 10 black leaders and informed them the president would sustain punitive measures against Rhodesia "through the transition period until elections were held," according to a spokesman for the blacks. The spokesman said the group endorsed Carter's move.

In a statement to Secretary of State Cyrus R. Vance, however, Carter said that he would be "prepared to lift sanctions when a British governor assumes authority in Salisbury and a process leading to impartial elections has begun."

The president added that "no party should have a veto over fair settlement proposals."

The senior State Department official, who asked not to be identified, later interpreted Carter's remark by saying that the final decision on chances all parties had to reach a fair and lasting settlement."

The official described as "eminently fair" the proposals advanced by Britian at the London conference.Asked what the administration would do if the conference breaks up without producing a settlement, he said "the president will review the decision on the basis of the final settlement proposals."

"Our assessment of what constituted a fair process would not be bound by other parties," he added.

Britain has announced it would lift sanctions once Rodesia "returned to legality" under a transition plan now being negotiated at the conference.

Muzorewa and Patriotic Front leaders have accepted constitutional proposals curtailing the power of Rhodesia's white minority. U.S. officials said both sides have made "some real concessions" in the current talks on the transition period leading up to "impartial elections."

If these concessions produce an accord, the conference would move into its final stage to arrange a cease-fire between the warring parties.