President Carter pledged yesterday to use "firm diplomacy" to deal with the newly disclosed presence of a Soviet combat brigade in Cuba and cautioned against "panic" and "exaggeration" at home.

The "status quo is not acceptable," Carter said, adding that Secretary of State Cyrus R. Vance soon will be negotiating with the Soviets about the once-secret brigade.

"We are confident about our ability to defend our country or any of our friends from external aggression," Carter said. "All of us . . . must respond not only with firmness and strength, but also with calm and a sense of proportion."

The president's remarks were addressed as much to the Senate as to the Soviets, although Carter did not mention the pending strategic arms limitation treaty, which is in new trouble in the Senate because of the situation in Cuba.

"This is a time for firm diplomacy, not panic and not exaggeration," Carter said.

The president was prompted to deliver his first public pronouncement on the Soviet brigade, according to a senior aide, by numerous statements by senators that SALT II would not be approved if the brigade were allowed to remain in Cuba.

The president particularly was concerned, the aide said, because several senators with hard-line positions on the issue were liberals and moderates who had been counted as SALT II supporters.

Carter declared that the presence of the Soviet brigade is "a very serious matter," and added:

"We do have the right to insist that the Soviet Union respect our interests and our concerns if the Soviet Union expects us to respect their sensibilities and their concerns. Otherwise, relations between our two countries will inevitably be adversely affected."

By publicly defining the goal of the negotiations with the Soviets only as changing the "status quo," the president carefully avoided specifying what diplomatic outcome is needed to ease the situation in Cuba. He specifically declined to answer questions from a reporter about whether the United States would insist on the removal of the Soviet combat force.

Carter repeated statements by Vance earlier this week that the Soviet force consists of 2,000 to 3,000 troops equipped with conventional weapons, including about 40 tanks and some field artillery pieces.

The president said there is evidence that the unit has been operating in Cuba "for some time, perhaps for quite a few years."

He then went on to describe what the force is not.

"It is not an assault force," he said. "It does not have airlift or seagoing capability and does not have weapons capable of attacking the United States.

"The purpose of this combat unit is not yet clear."

In recent years, the Soviets have used Cuba as a listening post to monitor telephone calls placed from the United States, but U.S. intelligence officials have said they do not believe that a Soviet brigade would be needed to defend the Soviet-built and Soviet-run intelligence network.

In remarks earlier yesterday, Zbigniew Brzezinski, Carter's national security affairs adviser, called President Fidel Castro's contention that Cuba is not aligned with the Soviet Union "fundamentally ridiculous."

"Castro is a puppet of the Soviet Union and we view him as such," Brzezinski said, adding "there isn't one instance" in which Castro has acted contrary to Soviet policy around the world.

In remarks to newspaper editors, Brzezinski said that economically, Cuba is "totally dependent" on the Soviet Union. He cited statistics, including $3 billion in Soviet economic aid supplied to Havana each year, one-quarter of the Cuban gross national product.

"Militarily," Brzezinski added, "Cuba is entirely dependent on the Soviet Union."

He concluded: "In effect, Cuba is an active surrogate for a foreign policy which is not shaped by itself, and is paid for this by economic and military support on a scale that underlines Cuba's status as a dependent client of the Soviet Union."

Hours before Carter spoke to reporters at the White House he discussed the troops situation with 14 members of Congress in the Cabinet Room.

Among those present was Senate Foreign Relations Committee Chairman Frank Church (D-Idaho), who first disclosed publicly that the presence of the Soviet combat brigade had been confirmed by U.S. intelligence.

Church said that Carter spoke of a "crisis atmosphere developing through the press, and of the way everyone has treated the matter."

Carter mentioned no names, Church said, but spoke critically of several proposed resolutions that would bar approval of the arms limitation treaty as long as the Soviet combat force remains in Cuba.

White House officials have been privately critical of Church's disclosure of the brigade, believing that he did so to bolster his hard-line credentials in Idaho, where he faces a reelection battle next year.

Carter told the congressional leaders that there was "no way to know how long" the Soviet brigade had been in Cuba, and said there was no "intelligence failure," in the delay in detecting it. Rather, he said, it resulted from a decision to allocate U.S. intelligence resources in other countries.

According to some who attended the meeting, Carter said the United States has a great deal of trouble getting good intelligence from Cuba. Sen. Barry M. Goldwater (R-Ariz.), one of those present, agreed, saying it was easier to get information from the Soviet Union. Goldwater is vice chairman of the Senate Select Committee on Intelligence.

He told Carter this was the Soviets' test of the president's mettle.

Majority Leader Robert C. Byrd (D-W. Va.) made the longest statement of the meeting, arguing that there was far from sufficient information available to declare that a crisis was at hand.

Several of those present said later that Carter received a generally sympathetic hearing from the group.

Reacting to Carter's later remarks, Sen. Richard Lugar (R-Ind.) said he wanted bold leadership. Sen. Jesse Helms (R-N.C.), suggested that Carter should have engaged in some "big stick" diplomacy.

"The president's speech was unsatisfactory," said Lugar. "We need a call for leadership and a sense that the president is prepared to respond . . . . The president reacted slowly, almost passively."

"I think the president ought to have said to the Soviets, calmly, 'Get the troops out or no SALT II,'" Helms said. "I think Jack Kennedy would have already told the Soviets, 'Get the troops out or no SALT II.'"

"The president needs to be given time for diplomacy to work, time to obtain accurate information. I don't think it's time for a crisis atmosphere," Byrd said, adding that SALT II should not be "held hostage" by the troops situation.

"I still have expectations for calling the treaty up this year," he said.

Church said he did not think he was exaggerating the situation by saying there is no hope for Senate approval of the treaty if the troops remain.

"I think it's important that the Russians should know from the chairman of the Foreign Relations Committee that these two issues are related," Church said. "The Senate will not ratify the SALT treaty while Russian combat troops remain in Cuba. We do have to draw the line on Soviet combat troops somewhere."