U.N. Ambassador Andrew Young begins a 10-nation tour of the Caribbean area today as part of what senior State Department officials call a major emphasis on a region long neglected by the United States.

Young's 12-day trip follows closely on the heels of recent visits to the Caribbean by such other rather highly visible representatives of the Carter administration as Rosalyn Carter, Secretary of State Cyrus Vance and Under Secretary of State Philip Habib.

"Andy Young is part of this effort because if somebody like him goes there it highlights our interest in the area," a senior department official said yesterday.

The aim, the official added, is to enable ranking U.S. policy-makers to become acquainted with the region's leaders an to gain some first-hand impressions of its institutions and problems. This knowledge, the official said, will be cranked into a still-developing, comprehensive new U.S. policy toward the Caribbean.

Prompting this rekindled interest, the official continued, is the Carter administration's feeling that the Caribbean - for reasons of geographic proximity, trade and investment - can have a major impact on events in the United States.

"Most of the islands are in very serious economic difficulties," said the official. "Their problems are in microcosm those of the entire developing world, but they are closer and more intimate to us. For that reason, we need to devote more attention and resources to the area, and we need to begin now."

The Caribbean's potential impact on the United States, he noted, goes far beyond such traditional security considerations as the presence within the region of Communist Cuba or the proximity of its sea lanes to the southern United States and the Panama Canal.

Because of high unemployment in the Caribbean islands, the area stands second only to Mexico as a source of illegible aliens in the United States. Looked at from another standpoint, the region provides two-thirds of the bauxite imports required by the U.S. aluminium industry and produced or refines 25 per cent of the oil entering the United States.

Yet, the official said, despite the obvious importance of what is sometimes called "American's third border," past administrations have given the more than 25 political entities in the Caribbean a low priority - "viewing them as simply one part of Latin America and giving inadequate attention to the Caribbean's own unique character and problems."

In taking about the region, department officials mean not just the string of islands stretching 2,000 miles through the Caribbean Sea from the Bahamas to Trinidad and Tobago. Instead, they are referring to a larger "Caribbean basin" area that also includes Mexico, Central America and the countries of northern South America.

The area, the officials says, cannot be measured with a single policy yardstick because it is made up of so many differing political and cultural strains. The nations of the regions vary in size from Mexico, the world's most populous Spanish-speaking country, to ministates of fewer than 100,000 inhabitants.

Politically, they range from such U.S. dependencies as Puerto Rico and the Virgin Islands to Washington's foremost hemisphere foe, Cuba. In between are some countries whose histories have been an almost unbroken chain of military dictatorships and some that have stable, viable systems of democracy.

In language and culture, the region is divided fairly evently between countries with a Spanish heritage and former British colonies that only recently have started to think of themselves as part of Latin America. There also are the cultural shadows cast by such other countries as France (Haiti, Martinique) and the Netherlands (Surinam, the Netherlands Antilles).

This diversity is illustrated by the list of countries on Young's itinerary: Jamaica, Mexico, Costa Rica, Guyana, Surinam, Trinidad and Tobago, Venezuela, the Dominican Republic, Haiti and Barbados.

If they have a common denominator, it is the potentially explosive combination of too many people and too few resources. Most of its people are impoverished, darked-skinned descendants of African slaves or Indian peasants.

For these reasons, the countries have identified themselves increasingly with the ideas and aspirations of the Third World.

So State Department officials regard Young - the Carter administration figure who has been most outspoken in championing the cause of the developing countries - as an important choice to symbolize Washington's concern with the Caribbean.

Young's talks in the individual countries are expected to deal largely with bilateral issues, such as increased U.S. financial aid to those governments struggling with troubled economics.

But some of Young's hosts - among them Jamaica's Michael Manley, Venezuela's Carlos Andred Perez and Guyana's Forbes Burnaham - wield considerable influence in the Third World.

Department officials hope that establishment of a personal relationship between Young and these leaders will be valuable in terms of U.S. relations with the Third World bloc in the United Nations and improvement of U.S.-Cuban relations.