President Carter is expected to inform Congress within a few days that he has decided to retain economic sanctions against Zimbabwe-Rhodesia but will reconsider his decision if the new African government in Salisbury significantly broadens its support, congressional sources said yesterday.

Carter's reported decision, which could be announced as early as today, would represent the administration's first important shift from its firm alignment on the Rhodesian issue with the regions's key African states, which oppose any restoration of trade until there is a negotiated end to Zimbabwe-Rhodesia's guerrilla war.

But Senate staff aides predicted that the reported compromise decision would not satisfy conservatives who have been pushing of an immediate end to sanctions against the biracial government headed by Prime Minister Bishop Abel Muzorewa.

Reacting to the reports that a qualified negative decision on lifting sanctions was imminent, Sen. Jesse A. Helms (R-N.C.) announced on the Senate floor that he again would seek Senate action to end the trade embargo as a sign of approval of Muzorewa's government.

Helms also was reported to be seeking an urgent meeting with the White House to try to arrange a last-minute compromise that would lift sanctions for one year and then require a review of the progress Muzorewa had made.

Administration officials refuse to comment on the reports of Carter's action, beyond confirming that a decision has been made.

At the daily State Department briefing, spokesman Hodding Carter said that the decision could be "either up or down," and then added, under repeated questioning, that "a 'no' decision could be modified by a promise to look at the situation again at some later date."

But Senior officials said that the spokesman's comment was not based on any specific knowledge of the decision, which is being held as a tightly guarded secret in the White House.

By 75 to 19, the Senate passed a resolution last month favoring the lifting of sanctions following parliamentary elections that brought Muzorewa to the top of a government still partially controlled by Zimbabwe-Rhodesia's small white minority.

The decision will come in a presidential determination required by 1978 legislation. If Carter determines that free and fair elections have been held and have produced a government that has made a good-faith effort to negotiate an end to the guerrilla war, then he is required to end the trade embargo.

Congressional sources said they had reason to believe that the most intense part of the White House debate over the compromise decision had been the setting of future conditions for the lifting of sanctions. "It boils down to how he says no, and whether he lists specific steps or retains complete flexibility," said one Senate source.

Rep. Stephen J. Solarz, (D-N.Y.), chairman of the House subcommittee on Africa and the leader of House efforts to turn back earlier attempts to lift sanctions, welcomed the reported decision, which he said "has the ring of truth."

Saying there was "a new situation in Zimbabwe-Rhodesia as a result of the elections," Solarz maintained that "it would be a serious mistake to precipitously lift sanctions" before several key events such as the Organization of African Unity summit in July and the Commonwealth meeting in August.

The decision by the new Conservative government in Britain to put off its action on sanctions was encouraging a new mood of "wait and see" on Capitol Hill, Solariz said, adding that the president would have "a reasonably good chance" of beating back immediate congressional action to overturn his decision if the White House made Zimbabwe-Rhodesia a high-priority issue.

Sanctions were enacted by Britain and the United Nations in 1965 when the white settler government declared unilateral independence from British colonial rule.