Cuba for the first time is practicing sophisticated amphibious landings, which pose a threat to the island states of the Caribbean, U.S. officials disclosed yesterday.

"Cuba cannot claim this capability is anything but offensive," said one administration official in terming the exercises "the first really sophisticated, integrated display of amphibious capability." The exercises utilize Cuban marines and amphibious vehicles.

About 400 marines, four light tanks and eight armored personnel carriers were spotted rehearsing amphibious assaults last week on the beaches near Mariel, about 25 miles west of Havana, officials said.

One U.S. specialist on Cuba who has studied intelligence reports on the amphibious exercise described it as "an important development." He said the marines, which the Cubans call naval infantry, provide President Fidel Castro with another weapon for exerting direct or indirect pressure on neighboring island states.

Given the small military forces available to most of those countries, the specialist said, 400 Cuban marines landing from amphibious ships with armored vehicles pose "a real threat."

"Cuba is developing the capability to project force all over the Caribbean," he said. "Castro clearly wants to become a major actor."

The amphibious ships, tanks and armored personnel carriers are part of the arms shipments from the Soviet Union to Cuba as an apparent payback for Castro's troops operating in such distant countries as Angola and Ethiopia.

The State Department estimates that the Soviets sent Cuba 66,000 tons of military weapons and equipment last year and the same amount in 1981. Administration officials said yesterday that it is too early to tell whether that level will be reached this year.

Cuba's amphibious capability includes two Soviet Polnocny-class amphibious ships, delivered in 1982, which can carry up to six tanks. Two such ships participated in the recent amphibious exercise, sources said.

The armored vehicles that landed from the ships standing off the beach were the Soviet-supplied BTR60P armored personnel carrier and the PT76 light tank. The integration of the Cuban marines and that armor in a sophisticated landing came as a big surprise to U.S. intelligence, sources said.

"This gives the Cubans the capability to project power anywhere in the Caribbean," said one official.

Officials would not disclose how they gathered the details of the amphibious exercise but it is no secret that the United States uses surveillance by satellites in space to help keep watch on Cuba.