President Carter and Turkish Prime Minister Bulent Ecevit have agreement on a plan for lifting the U.S. arms embargo that has seriously strained the NATO alliance, informed sources said yesterday.

Under the plan, the sources said, Carter will ask Congress to repeal the 3-year-old embargo in exchange for Ecevit's dropping his threat to close U.S. bases in Turkey permanently and loosen his country's ties with NATO.

As a concession to congressional supporters of a continued embargo, the Plan also calls for scropping a proposed four-year $1 billion defense cooperation agreement providing U.S. military aid to Turkey.

Instead of asking Congress to approve the four-year pact, the sources said, the administration will seek to make military grants and sales credits available to Turkey on a year-to-year basis. That would enable Congress to review Turkish aid requests each year when it votes to the administration's foreign assistance budget.

The sources said agreement on the plan was worked out this week during discussions in Ankara between Ecevit and Deputy Secretary of State Warren M. Christopher, whom Carter had sent to Turkey Monday.

If the details of minor points insisted on by Turkey are acceptable to the White House, the president is expected to announce the agreement immediately after his return next week from Africa, the sources said.

Reports from Ankara yesterday quoted Ecevit as telling reporters: "I am in a position to say that we have come to a hopeful and significant stage to smooth out the relationship." He refused, though, to discuss any details.

If the agreement goes through, the sources said, it is expected to lessen greatly the tensions that have troubled U.S.-Turkish relations since Turkey's 1974 invasion of Cyprus.

Turkey's continued occupation of 40 percent of the island nation, which has an 80 percent Greek population, angered the sizable Greek-American community and its supporters in Congress.

In 1975, the so-called Greek lobby won congressional passage of a U.S. arms embargo against Turkey. Turkey retaliated by closing down most of the U.S. bases there - bases that the Defense Department regards as vital to the effective defense of NATO's southern flank in the Mediterranean.

Since then, the embargo has been modified to allow Turkey to buy up to $173 million in arms annually. In 1976, the Ford administration negotiated a defense agreement that called for reopening the bases, ending the embargo and allowing Turkey $1 billion in military aid over four years.

However, Congress shelved the agreement, pushing the problem over to the Carter amdinistration. Although Carter has been under pressure from his military advisers to seek an accommodation with Turkey, he has hesitated until now because of the domestic political implications involved in antagonizing the Greek-American community.

Last year, Carter decided not to seek congressional approval of the Turkish defense agreement until Greece and Turkey showed signs of progress toward a solution of the Cyprus problem.

But, that linkage has been rejected by Ecevit, who took office in January. During recent weeks, he has threatened that, unless Washington acted quickly, Turkey would withdraw permanently the U.S. rights to bases there and begin removing Turkish military units from NATO control.

To undescore his threat, Ecevit begen dropping hints that he would boycott a summit meeting of NATO leaders planned for Washington on May 30 and 31.That, and such other signs of his increasing restiveness as an announcement that he is planning a trip to Moscow, caused Carter to send Christopher on this week's hastily arranged mission to Ankara.

In discussing the agreement worked out there, the sources said it was not clear whether the plan to continue Turkish military aid on a year-to-year basis will give it the same level of assistance called for by the defense agreement.

Under the defense pact, the United States would have provided Turkey with military grants and credits of $250 million annually for four years.

The Carter administration's military aid budget requests to Congress for the 1979 fiscal year make no provision for grants and ask Turkish sales credits of $175 million.

As of now, the sources said, it is doubtful that the administration will seek to increase this fiscal 1979 request, even if the proposed agreement proves acceptable to Congress.

As to ensuing years, the sources said, it is doubtful that the administration will beef up its annual requests to a level approximating the $250 million that Turkey would have received under the defense pact.