U.S. intelligence has determined that the Soviet Union is building a new pier at the Cienfuegos naval base in Cuba, but Carter administration officials said last night they do not know whether the facility could be used for offensive purposes such as servicing Soviet nuclear submarines.

The officials stressed that, at this point, there are no grounds for regarding the pier construction with the same concern triggered by the discovery in August of a Soviet combat brigade in Cuba.

In fact, the officials added, there is no evidence to suggest that the construction violates the understanding reached by the two superpowers in settlement of the 1962 Cuban missile crisis. Those agreements are supposed to bar the Soviets from introducing offensive weapons onto the island.

They said the construction, which was detected by increased U.S. surveillance of Cuban in the wake of the furor over the Soviet brigade, is being watched closely and has been reported to key members of Congress dealing with security affairs. They added that Secretary of State Cyrus R. Vance also discussed the matter in general terms at a meeting Monday with Hispanic-American community leaders.

However, despite the cautions tone taken by the officials, fallout from the controversy over the Soviet brigade -- coupled with a companion dispute about Cuba as a potential base for servicing Soviet subs and warships -- seems certain to unleash new questions about Soviet military activities in Cuba.

That was underscored last night by Sen. Richard B. Stone (D-Fla.); who revealed the presence of the Soviet brigade last summer at a time administration officials were insisting they had no evidence to support such charges.

In a telephone interview, Stone, who has been briefed on the matter, said the construction is of " major proportions" and has been under way for several months. The senator added that he had more information but could not reveal it because it was clasified and went beyond the facts that emerged yesterday.

But Stone, who repeatedly has raised questions about Soviet submarine visits to Cuba, said the matter raises questions "of concern about whether the Soviets are attempting to build a major submarine base in Cuba."

He continued: "It is time to get this situation clarified, both in terms of clearing up ambiguities about the extent to which facilities in Cuba can be used by Soviet warships, including those with nuclear armaments, and in terms of making clear we will not tolerate a major base there operated by the Soviets or their Cuban allies."

The U.S.-Soviet understandings about Cuba, which have never been made fully public, have been a subject of controversy over the years in regard to naval matters.

In 1970, after concern developed that he Russions were building a submarine base at Cienfuegos Bay, President Nixon announced that port calls in Cuba by Soviet submarines and ships would not violate the agreements, but the servicing of such vessels "either in or from Cuba would be a violation."

The pier whose construction has now been detected is in addition to another recently built at Cienfuegos Bay under Soviet supervision. Although it apparently was ready for use when Soviet ships were deployed in the Caribbean last spring, the Russian vessels did not visit Cuba.

That is believed to have been the result of U.S. press reports calling attention to the possibility that a violation of the 1962 agreements and the 1970 Nixon interpretation of the accords might occur. The Carter administration is understood to have warned the Soviets at the time that a visit by the Soviet ships to Cienfuegos could be considered by a provocation in this country and lead to strained relations.

However, there has been continued speculation about whether Soviet ships and subs will continue to avoid going into Cienfuegos Bay. As a result, revelation that a second Soviet pier is under construction seems guaranteed to regenerate debate about what the Soviets are up to at the Cuba facility.

According to administration sources, the new construction also involves several large buildings or sheds, and some sources said that this seems like a lot of construction to handle the two Soviet subs known to be at the Cienfuegos base.

One of these vessels, they said, is an overage submarine used by the Cubans for training purposes. They said the other, manned by a Soviet crew, is a diesel-powered attack submarine armed with conventional torpedoes but not missiles.