U.S. intelligence has detected crates of new Soviet aircraft at an airfield outside Havana, officials said yesterday, presenting the Reagan administration with a dilemma on how to react to this event.

If President Reagan or any of his Cabinet members sounds the alarm about the new shipment of planes to Cuba, they may well inspire demands from conservatives to do something about it.

This was the problem President Carter encountered in October, 1978, after then-Defense Secretary Harold Brown warned him that the Mig23s the Soviets were sending to Cuba might be a violation of the 1962 Washington-Moscow "understandings" that ended that year's missile crisis.

The Soviets in those understandings promised to refrain from sending offensive weapons to Cuba. The Mig21 and Mig23 fighters sent to Cuban President Fidel Castro since 1962 have been described by Washington and Havana as defensive, meaning they are armed to knock down enemy planes rather than bomb distant targets.

It cannot be learned from satellite and spy plane photography alone whether a Mig has been wired for offensive operations, perhaps all the way up to dropping a nuclear weapon. But the Reagan administration has continued to send planes over Cuba equipped with devices for detecting nuclear weapons and has found none, sources said yesterday.

However, the six to eight crates of aircraft recently spotted in Cuba present a problem to Reagan politically. Their shipment signals that the Soviet Union is, at a minimum, continuing to modernize the Cuban air force and that, at a maximum, it may be sending the Cubans a bombing version of the so-called Mig23 Flogger, the D model designated as the Mig27, which could represent a violation of the 1962 understandings.

Nobody knows for sure what is in those crates. So, as of last night, the administration was preparing a short statement designed to take note of the shipment without setting off alarm bells the president is not prepared to answer.