President Carter, unable to obtain a negotiated end to a Soviet "combat brigade" in Cuba, last night unveiled a series of regional and worldwide countermeasures to be taken by the United States.

In a concerted attempt to end a month of political controversy and international uncertainty, Carter presented a balanced combination of assurances from the Soviet Union, harsh words about its Cuban "satellite" and unilateral U.S. actions to compensate for the continuing Soviet presence.

Carter also renewed his fervent appeal for Senate approval of the strategic arms limitation treaty (SALT II), which has been imperiled by the troops-in-Cuba dispute. He depicted steady U.S.-Soviet relations to preserve the peace and ward off nuclear conflict as far more important than 2,000 to 3,000 Soviet combat troops in Cuba, and said the breakdown of this dialogue would be "the greatest danger to American security."

The announcement of actions to counter the Soviet military force was a tacit admission that three weeks of diplomacy had failed to bring about a negotiated settlement of the dispute.

The president disclosed a series of Soviet assurances, obtained through an exchange of confidential messages with Soviet President Leonid Brezhnev and negotiations with Soviet Foreign Minister Andrei A. Gromyko, that appeared to limit the future activity and capability of the "combat brigade" (in the U.S. description) or "training center" (as the Soviets and Cubans say it is).

Nonetheless, the United States clearly had been unable to obtain withdrawal of the Soviet brigade, as Carter told lawmakers he had sought, or even elimination of the unit's combat capability through transfer of its key personnel and major equipment.

Carter's countermeasures included increased surveillance of Cuba and its periphery, diplomatic assurance and economic aid to its Latin American neighbors and stepped-up maneuvers that will land additional Marines at the U.S. base at Guantanamo Bay, Cuba, for a temporary stay.

Carter also ordered an increase in U.S. intelligence activities worldwide and a step-up in the formation of a proposed Pentagon quick-reaction force to react to trouble anywhere.

The countermeasures appeared to be designed to avoid posing a physical threat to Moscow or Havana, and to avoid a military response on their part.

"We do not face any immediate, concrete threat that could escalate into war or a major confrontation," the president said.

"I have concluded that the brigade issue is certainly no reason for a return to the Cold War," he said at another point in his 20-minute address, which was delivered in a dispassionate manner without rhetorical flourishes.

The Soviets, who were not informed in advance of the measures the president announced, may be more upset by a decision not mentioned in the address but leaked to several publications last weekend and officially confirmed yesterday. This is the forthcoming visit by Defense Secretary Harold Brown to the Soviet Union's neighbor and archrival, the People's Republic of China. The first trip of a U.S. military chief to Peking since the communist takeover may bring significant steps toward a U.S.-Chinese military relationship.

Carter continued to take a tough line against Fidel Castro's Cuba. He did not repeat the word "puppet," which brought a strong reaction from Havana when Carter hurled it at a New York town meeting last week. But the president said Cuba is dominated by the Soviets and "automatically follows the Soviet line."

Carter did not describe in detail the intelligence that led to the U.S. conclusion that a Soviet combat brigade is in Cuba. However, a high-level intelligence official told a press briefing yesterday at the White House that the brigade contains 2,600 men, 40 tanks and 60 armored personnel carriers.The official said the brigade is organized into three infrantry and one tank battalion and is garrisoned at two sites.

The mission and structure of the brigade have changed twice since the Cuban missile crisis of 1962, once when it was all but disbanded and more recently when it uas enhanced, the official said.

The brigade is not an assault force, has no airborne or seaborne capability, and "presents no direct threat to us," the president said.

Nonetheless, he described it as "a serious matter" to the United States, saying "it contributes to tension in the Caribbean and Central American region."

Disclosing for the first time the results of three weeks of diplomatic negotiations, Carter listed assurances that he said were given to him "from the highest levels of the Soviet government."

Reporters were told these assurances grew out of a message to Carter from Brezhnev last Thursday, the same day as Secretary of State Cyrus R. Vance's final meeting with Gromyko in New York. Brezhnev was responding to a direct message from Carter two days earlier.

One key assurance was that the Soviets, who insist that their force is a training center, "will not change its function or status." Carter added that the United States "understands" that the Soviets do not intend to enlarge the unit or give it additional capabilities, but he made no claim that the Soviets agree to this U.S. understanding.

Another key assurance cited by Carter was a Soviet statement that its personnel in Cuba "are not and will not be a threat to the U.S. or to any other nation."

Carter called these Soviet assurances "significant," although the continuing disagreement with the Soviets over the nature and mission of the force tended to diminish the effect of this statement.

Saying he would not rest on Soviet assurances alone, the president announced several countermeasures for the Latin region:

Increased surveillance of Cuba. No details were given, but there was speculation this could involve a resumption of the regular spy plane overflights that were canceled by Carter as a gesture of goodwill at the time of his inauguration as president.

Diplomatic assurances to Latin nations that "the United States will act in response to a request for assistance in meeting any . . . threat from Soviet or Cuban forces."

Establishment of a permanent Caribbean Joint Task Force headquarters at Key West, Fla., to replace an interim task force.

Expanded military maneuvers in the region on a regular basis. For the foreseeable future, these will involve about 3,500 U.S. military personnel, including Marines.

An unspecified increase in U.S. aid to Caribbean nations to meet "economic and human needs" and help countries "resist social turmoil and possible communist domination."

Measures announced by Carter outside the region were:

An increased capability of "rapid deployment forces" assigned by the Pentagon to meet contingencies anywhere in the world.

A continuing reinforcement of the U.S. naval presence in the Indian Ocean.

Enhanced worldwide intelligence capability to monitor Soviet and Cuban military activities.

Speaking on his 55th birthday, the president ended his speech with a strong plea for the nuclear arms treaty with the Soviets, a top priority throughout his administration.

The rejection of SALT II would leave U.S. allies "confused and deeply alarmed," and would "seriously compromise our nation's peace and security." he said. Failure of the treaty could cause every U.S.-Soviet dispute to take on ominous dimensions and even aspects of nuclear confrontation, he declared.