Intelligence reports coming from Grenada after the U.S. invasion show a stronger and larger Soviet-Cuban military and diplomatic presence than expected, but they may fall short of supporting President Reagan's assertion that the Caribbean island was "being readied as a major military bastion to export terror," according to administration and congressional sources.

After the Senate Intelligence Committee was briefed yesterday by CIA Director William J. Casey, a Republican member of the committee said that the large, combat-ready Cuban construction force and extensive antiaircraft batteries camouflaged around the Point Salines airport show the Soviets and Cubans had established a permanent base of operations.

But the senator added that the administration's case that the island was being readied to export revolution and terror in the eastern Caribbean was "inferential and circumstantial" at this point.

Pentagon officials said yesterday they believed there are about 1,100 Cubans on the island, 600 of whom have been captured by U.S. forces who landed on Grenada Tuesday morning. The CIA has sent five interrogators to Grenada to begin screening the captured Cubans, a senior official said.

The official said the discovery of the well-established Cuban presence provided U.S. intelligence analysts with a unique opportunity to study closely the military management and organization of a small state under Soviet and Cuban influence.

The Cuban construction crews turned out to have been equipped for combat much like U.S. Navy Seabees (construction battalions), according to the official. In their barracks on the southern end of Grenada, the official said, American forces found special hooks by the Cubans' bunks from which to hang their AK47 assault rifles.

The Cubans were said to have high technology Soviet and Cuban communications gear and weaponry. One senior official said a Cuban general was believed to have been on the island recently in a command or advisory capacity.

The Pentagon has highlighted the presence on the island of a Cuban colonel as an indication of the importance placed on the military base.

Several sources, including a Pentagon official and a member of the Senate Intelligence Committee, said yesterday that the Cuban colonel, Pedro Tortolo Comas, was sent to Grenada the day before the invasion "to lead the resistance." By the time Tortolo arrived, the impending invasion was being widely reported in the Caribbean. Political leaders there were debating whether to endorse the invasion plan by some eastern Caribbean countries.

A Reagan administration official said the volume of weapons found in Grenada by invading U.S. forces far exceeded the defensive needs of the island and the only logical conclusion was that the island was being prepared as a staging area by the Soviets and Cubans.

"The raw military photos alone and the raw numbers, including millions of rounds of ammunition, make the case," this official said.

Contrary to earlier reports, a Senate intelligence staff source said a Cuban ship named the "Vietnam Heroica" did not resupply the Cuban troops during the weekend before the invasion. The supply ship had been in a Grenada harbor since Oct. 17 or Oct. 18, this source said, before the death of Grenada's former prime minister, Maurice Bishop.

U.S. forces have found 49 Soviet bloc diplomats and dependents in Grenada. In addition, U.S. officials said they were surprised to be told that 20 North Koreans, 10 East Germans and four Bulgarians were taking refuge in the Soviet mission in Grenada.

This is considered large for a country that size and is certain to include intelligence officers, said a senior administration official, who added that the 20 North Koreans almost equal the number of North Koreans in Nicaragua supporting the Soviet-Cuban presence there.

At the closest U.S. Embassy in Bridgetown, Barbados, there have been as many as 40 to 50 American diplomats with a total staff of 155 personnel, according to Sally A. Shelton, a former ambassador to Barbados. The embassy represents U.S. interests in Barbados and a number of other Caribbean countries.

During yesterday's Senate Intelligence Committee briefing, according to one Republican senator who attended, Casey compared Grenada and Nicaragua, noting that the mix of Soviet bloc diplomats and advisers seems roughly the same in each of the two countries.

Though Casey apparently did not mention it to the Senate committee yesterday, a senior administration official said the CIA has indications that a Soviet-backed assassination team was involved in killing former prime minister Bishop last week, six days before the U.S. invasion.

It was reported from the region last week that Bishop and a number of his aides were killed after a crowd of supporters freed him from house arrest Oct. 19. He reportedly was executed after being seized outside Fort Rupert on the island by Grenadan troops then under the command of Gen. Hudson Austin.

The administration official declined to reveal the basis for information about a Soviet assassination team. The official cited this information as an "important intelligence byproduct" being gathered in the wake of the invasion.

Another senior administration official, who monitored intelligence reports in the White House this week, said there was no mention of the alleged Soviet assassination team in the reporting. However, the official said he heard verbal reports of such a team having been sent to Grenada, but took the unsourced reports to be of rather low reliability.

A senior administration official also said Soviet and Cuban military documents and secret files seized after Tuesday's invasion were booby trapped with explosives and had to be defused. Officials said U.S. military forces were able to take possession of most of the documents safely.

One Pentagon official said the documents did not contain any written contingency plan by the Cubans or Grenadans to seize hostages to forestall a U.S. invasion. The official said it was assumed here in advance of the invasion that the Cubans might take hostages. The documents are still being translated and analyzed.

Several sources familiar with Casey's briefing said the CIA received very little criticism from the senators yesterday for underestimating the Soviet and Cuban presence on Grenada. "There is general agreement that it is difficult to get good intelligence on an island when the Cubans are in charge," said one source.

Casey reportedly also made it clear that he favored the invasion by a force of U.S. Marines and Army Rangers and feels it will send an important message to the Soviets and Cubans that their presense in Nicaragua is not accepted, that Nicaragua is not immune from U.S. intervention, and that the the United States is willing and able to strike to protect its interests in the hemisphere.

Congressional sources said that Reagan recently signed a new presidential "finding" to justify CIA covert activity in the Caribbean to "support democracy," conduct "counterterrorism" operations and fight drug trafficking.

One congressman said the finding proposed no new funds for such operations, but added that the finding was couched in such vague language that "you could drive a truck through it." A Senate source said the finding will allow the CIA to support friendly political organizations.