The Soviet Union has sent Cuba a more advanced fighter plane than any in that island nation's inventory, the Pentagon said yesterday.

But spokesman Henry Catto Jr., in confirming deliveries of an unspecified number of the Soviet Mig 23 Flogger B fighters, stressed their defensive rather than offensive nature.

"It involves an upgrading of Cuban defense capabilities," Catto said in an apparent effort to avoid stirring up protests that the Soviet Union was breaking its pledge not to send Cuba offensive weapons which would threaten the United States. That pledge reportedly was part of the still-secret understandings that came out of the U.S.-Soviet missile crisis of 1962.

"Cuba began receiving the B version of the Mig 23 earlier this year," Catto said at a Pentagon news briefing. "So far they've only received a few, but we have no idea how many eventually they will get. We don't consider the 23 B to be the latest example of Soviet air defense technology because the system is a decade old. These will probably be used as replacements for the older Mig 15s and 17s."

The Pentagon spokesman added that the Flogger is designed for air-to-air combat rather than bombing, a role that is considered defensive rather than the offensive role of bombing and strafing. The Soviets also are believed to have sent the more advanced Apex and Aphid air-to-air missiles to Cuba, along with the more modern jets, but Catto did not confirm this.

Asked if delivery of the Mig 23 Bs violated the 1962 understandings, Catto replied: "Not to the best of my knowledge. I would refer that kind of question to the State Department."

The Soviet Union until this year had sent Cuba its conventional export version of the Mig 23, an airplane less sophisticated than the model B, which is a top-of-the-line Soviet fighter. The Central Intelligence Agency this winter spotted crates of new Soviet aircraft in Cuba. CIA officials said at the time they did not know what type of planes were inside. Recently, perhaps on the basis of photos of the assembled planes, the CIA concluded that some of the new planes are the Flogger B with improved radars that can look down as well as straight ahead.

It is likely the Soviets would deliver at least a dozen of the hotter Mig 23s to form a squadron. Regardless of the actual number, the Flogger B shipments suggest that Moscow had decided to continue modernizing the Cuban military despite Reagan administration protests.