Deputy Secretary of State Kenneth W. Dam said yesterday that the United States has captured a "treasure trove of documents" in Grenada, including an agreement for the Soviet Union to ship weapons through Cuba to the ousted Grenadan regime.

Dam said the documents also include separate agreements for Grenada to receive supplies from communist Cuba and North Korea. To further bolster the administration's contention that Grenada was being turned into a base for insurgency in the Caribbean, he disputed media reports that many of the stockpiled arms found by U.S. forces were "antiquated," and he asked:

"Why would the Grenadan militia," which had an estimated 1,200 men, "require 18,000 uniforms?"

Dam, interviewed on "Face the Nation" (CBS, WDVM), was one of three senior administration officials who went on television yesterday to defend U.S. intervention as necessary to rescue U.S. citizens and block a Soviet-Cuban plan to use the tiny eastern Caribbean island as a center for aiding subversion and insurgency in the region.

The other two officials were U.N. Ambassador Jeane J. Kirkpatrick and Lawrence S. Eagleburger, undersecretary of state for political af- fairs.

Although the Defense Department earlier had scaled back by about 300 its estimate of the number of Cubans on the island, Dam and the others insisted that, when captured equipment and documents are analyzed and the results made public, administration charges will be validated.

The three officials spoke as the Pentagon announced that 16 Americans died in the fighting that began last Tuesday. The revised casualty report increased the number of American dead by five and reported 77 wounded and three missing. Of the five new deaths, four men had been listed as missing.

As Pentagon officials confirmed that the fighting now has largely ended, attention began to focus anew on when the approximately 6,000 U.S. troops involved in the invasion will be withdrawn and what steps will be taken to create an independent government for the 110,000 people of Grenada.

Prime Minister Tom Adams of Barbados, one of six Caribbean nations joining the United States in the intervention, said the U.S. military role will be finished when Grenadan Governor General Paul Scoon completes plans for an interim government.

Adams, interviewed from Barbados on "This Week With David Brinkley" (ABC, WJLA), said he expects Scoon to announce his arrangements this week.

U.S. officials have spoken only in vague terms about their expectation for Grenada's political future. However, they are known to believe that the situation should follow the pattern used after the 1965 U.S. intervention in the Dominican Republic.

That involves a period of interim civilian government while arrangements are made for free elections supervised by an independent body such as the Organization of American States or the United Nations.

The officials have acknowledged that some outside military presence will be required during the transition, but they also believe that U.S. troops can be withdrawn and the policing job turned over to a force formed by Grenada's Caribbean neighbors. There also have been suggestions that the task be given to a force recruited from Britain and other Commonwealth countries.

Adams said that British troops are not needed in Grenada but that Britain should continue to help train a police force in the former British dependency. Adams, who toured the island Saturday, said he believes that there is little tension or likelihood of renewed strife and that most of the Grenadan militia "has disbanded itself and is preparing for the return of civil conditions."

One unanswered question about the proposed elections is whether they would be open to parties that the United States and Grenada's neighbors regard as Marxist.

Prime Minister Maurice Bishop, executed Oct. 19 in a power struggle that led to the invasion, was very popular with Grenadans despite his self-described "radical leftist" leanings, and fully open elections could return to power a government with similarly pro-Cuban attitudes.

Adams said "it would be all right with me" if Marxists or other radicals gained office through elections instead of resorting to a coup as Bishop's "New Jewel Movement" did in 1979. But he added:

"I'm sure the American people will elect a Marxist government before the people of the eastern Caribbean."

Dam and Eagleburger also insisted yesterday that there is no plan to invade Nicaragua, whose Cuban-oriented government is at odds with the United States. "No, flatly no," Dam said and, asked if there might be a contingency plan, he replied:

"I have no idea what may be in some drawer somewhere in the Pentagon. The administration is not talking about, not planning any invasion of Nicaragua."

Meanwhile, a four-day airlift of civilians in Grenada ended over the weekend, with all American students at St. George's University School of Medicine accounted for. A flight arriving late Saturday in Charleston, S.C., brought to 677 the number of civilians evacuated since Tuesday.