Soviet leader Mikhail Gorba-chev will sign a friendship treaty with Cuban President Fidel Castro Tuesday sealing three decades of alliance.

The treaty of friendship and cooperation is intended to show the world, and the United States 90 miles away, that unity prevails between the two communist allies despite their choosing different roads to solve their economic and social problems.But the substance of Gorbachev's first visit to an island once seen as so strategically important to Moscow that it warranted the deployment of Soviet nuclear missiles was expected to come in his afternoon speech at the National Assembly of Popular Power.

Gorbachev, on his first trip to Latin America, was expected to outline Moscow's growing interest toward a region long regarded as Washington's "back yard."

The first round of talks Monday centered on Latin America's problems. A Soviet spokesman said they agreed that its huge foreign debt of $420 billion to industrialized nations amounted to "effective robbery."

Castro contends that the developing world's external debt is unpayable and should be written off, but Soviet spokesman Gennady Gerasimov gave no indication of whether that drastic approach was shared by Gorbachev.

Gorbachev said in December that Moscow was prepared to order a moratorium of up to 100 years on debt servicing by the least developed nations, which would include Cuba.

A beaming Gorbachev expressed delight at being "in the embrace of friends." But diplomats described Sunday's welcome as "rather tepid by Caribbean standards."

With the talkative Castro explaining at length every technical detail, Gorbachev visited Expocuba, a permanent exhibition center of the economic, industrial, scientific and social achievements of 30 years of socialism.

Tuesday morning he visited a microbrigade of volunteer construction workers and then see examples of Cuba's advanced health system - including the state-of-the-art $110 million bio-technology institute.

Gorbachev's wife, Raisa, lit up Havana's Revolution Square district with a little curbside diplomacy. She visited a showpiece day-care center, one of more than 100 built in the Cuban capital over the past two years.

Later she saw the Hemingway museum, the mansion-like residence just outside the capital where American writer Ernest Hemingway, revered both in the Soviet Union and Cuba, spent many years.