Defense Secretary Harold Brown said yesterday there is "a reasonable chance" of reaching a nuclear arms control pact with the Soviet Union this year, partly dependent on the total "climate" of U.S. Soviet relations.

The Soviet Union has made "some movement on a number of issues." Brown said, and "we've clarified our position on a number of issues."

But the pattern of negotiations with the Soviet Union, he reminded reporters at an Overseas Writers luncheon, always leaves some of the most critical issues to the final hours, or minutes, of bargaining.

Therefore, he said, the ultimate result is affected not only by how positions of the two sides evolve, but "it also depends inevitably on the external climate."

This is "not because there is any, if you will excuse the expression 'linkage' with specific other subjects." Brown said, "because if a SALT (Strategic Arms Limitation Talks) agreement is in our interest, then we should go ahead and consummate it."

It is because the "timing, public reaction, congressional reaction - all of which are important to SALT agreement - are inevitably affected by other conditions," he said.

In other words, Brown again was underscoring the Carter administration's view that the totality of U.S. Soviet relationships can affect what happens at the conclusive stage of SALT, despite Soviet indignation about the invocation of direct or indirect 'linkage with SALT.

Brown gave a different account, however, than a White House official presented last week about what is happening in one area of high sensitivity for U.S.-Soviet relations, the Horn of Africa.

"I am not prepared to say," he told reporters, that Cuban troops that have been operating in Ethiopia "are moving into Eritrea, but I don't want to rule out that possibility in the near future."

t would be a significant change in the use of Cuban troops in Ethiopia, Brown said, if they enter the internal Ethiopian war against the secessionist province of Erithea. Cuban troops and Soviet military Advisers helped Ethiopia to rout intruding Somali forces in Ethiopia's Ogaden region last month.

During President Carter's trip to Africa last week, reporters were told that some of the Cubans brought into Ethiopia to fight the Somalis "are being redeployed against the Eritreans."

Only "perhaps . . . small-scale units" are in a combat role in Eritrea so far, the high official said, but "there is some indication - it isn't conclusive yet - that they will be massively employed."

That official put the number of Cubans in Ethiopia at 16,000 to 17,000 - the high side of an estimate that the State Department had put as "at least 15,000." Brown yesterday spoke of "probably 15,000 or more" Cubans in Ethiopia.

State Department officials earlier this week acknowledged "discrepancies" in what officials were telling reporters and said they would only state "it's a murky situation."