The Pentagon announced yesterday that more than 6,000 U.S. troops are participating in the invasion of Grenada. The total is more than twice that previously disclosed.

About 700 Army Rangers were to be withdrawn last night, and 500 Marines may leave soon, but more than 5,000 U.S. troops must remain on the Caribbean island to track down and "neutralize" Cubans and "hard-core, trained Grenadans" still at large, according to Adm. Wesley L. McDonald, the Atlantic commander-in-chief who is running the military operation.

At a Pentagon news conference yesterday, McDonald said U.S. forces found about 1,100 Cubans in Grenada and captured 638. Between 300 and 350 other Cuban fighters and an unknown number of Grenadan soldiers escaped into the hills of the 133-square-mile island as U.S. forces overran their positions, he said.

McDonald declined to provide casualty figures for Cubans, Grenadan soldiers or civilians and also declined to predict how long the operation will last, although he said "all major military objectives" have been met. He said that 11 U.S. troops have been killed and 67 wounded and that seven are missing.

McDonald also said he could not rule out the possibility of establishing a U.S. military base in Grenada.

The administration continued to escalate its claims of Cuban involvement in Grenadan affairs prior to the U.S. operation. Documents captured during the invasion spelled out a "long-range plan for Cuban intervention" that included deploying an additional 4,341 troops there, McDonald said.

"The way this is geared is for the Cubans to come in and take over the island, which they had already started to do," McDonald said. "The Cubans were planning to put their government into Grenada."

The administration has not released copies of any documents found in Grenada.

The United States launched a pre-dawn invasion of the island Tuesday following a violent coup Oct. 19 that replaced a leftist government with a military junta.

President Reagan said the surprise attack was intended to assure the safety of more than 1,000 U.S. citizens in Grenada and as a response to the request of neighboring Caribbean countries' leaders, who said they feared an unstable regime in Grenada might threaten them.

The Pentagon said the invasion force included 700 Army Rangers from bases in Georgia and Washington state, 500 Marines from offshore ships and 1,500 soldiers from the 82nd Airborne Division at Fort Bragg, N.C. A small number of Seals, the Navy's special commando unit, also took part, although the Pentagon never officially acknowledged their participation.

Those figures had not been updated by the Pentagon, which has controlled the flow of news tightly all week. On Wednesday, when reporters asked Defense Secretary Caspar W. Weinberger whether he expected to keep the troop level at about 3,000, he said:

"I think that, so far as we know, what we have now will be sufficient, but, again, nobody is making any predictions."

But military sources said planeloads of soldiers and equipment from the 82nd Airborne were flying almost continuously Wednesday and yesterday from North Carolina to Grenada and a staging area in Barbados, about 150 miles away.

Officials clarifying Adm. McDonald's remarks yesterday said 5,000 troops from the 82nd Airborne, 500 Marines and several hundred more support personnel are in Grenada, in addition to the Rangers scheduled to leave last night.

The officials said the Marines probably will depart within a week, leaving an occupation force of about 5,500 Army soldiers. McDonald said the 22nd Marine Amphibious Unit in Grenada, with its five ships and 1,800 Marines, may still reach Lebanon on schedule next month to replace Marines serving there.

But McDonald declined to predict when the remaining soldiers will leave Grenada. Weinberger said Wednesday that he hoped they would leave within "days or weeks," while McDonald modified the prediction yesterday to say he hopes the fighting will end within days or weeks.

Since Wednesday afternoon, the number of Cubans reported in custody has risen only slightly, and McDonald said he believes most uncaptured Cubans have melted into the "dense tropical growth."

"They're fighting delaying actions," he said. "As those places are being overrun--and I would say, with a restraint of force--they are then, when they are able, disappearing into the hillsides.

"That," he continued, "will present a problem."

McDonald said the invading force has not geared up to fight guerrilla actions because the United States underestimated the number and military strength of Cubans on the island. He said U.S. forces must track down the Cubans and remaining elements of Grenadan resistance before leaving.

"I think you have to look at the Grenadans who were loyal to the government that was in existence before this started happening, to identify the people who are the hard-liners," he said.

"I think the identification process is going to be one that's very difficult for us to continue to pursue, but one that we've got to do because we cannot afford the withdrawal of all of the forces and allow an insurgency government to reappear."

A Pentagon official who asked not to be named said Wednesday that he does not believe Cuba will cease trying to infiltrate and disrupt U.S. activities on Grenada, even after Cuban troops have surrendered or been killed. As a result, he said, U.S. forces or some other strong military contingent might have to remain on the island "indefinitely."

"I would certainly hope that we would not have to stay there very long," McDonald said. "If we can identify those hostile Grenadan and Cuban forces and get those forces neutralized, then we will get out of there . . . ."

Before the fighting began but "when it appeared U.S. intervention was likely," McDonald said, Cubans "took over control of the island." The Point Salines airport, where the Army encountered the stiffest resistance, was "Cuban-controlled and sealed off from Grenadans," he said.

Other sources said Rangers parachuting into Point Salines and the Army transport planes carrying them encountered much stronger resistance than expected.

"They went in there thinking it was going to be a breeze, and they found themselves in a very tough fire fight," one Pentagon official said. "We were lucky we didn't take more casualties than we did."

The official said the Rangers' parachutes could have been a perfect target for snipers, although the invaders were aided by darkness and confusion created by the situa- tion. "Even once we took the airport, it wasn't easy," he said. "They were very well dug in all around the area."

The Seals also reportedly had difficulty with their landing, sources said. High seas made their approach difficult, and one unit was reported pinned down by gunfire from an armored personnel carrier ashore. It could not be learned yesterday what became of that unit.

McDonald's briefing, eyewitness accounts and information from other sources yesterday indicated that U.S. forces used heavier firepower than Pentagon officials previously implied.

A7 Corsair light attack jets from the aircraft carrier USS Independence repeatedly bombed and strafed antiaircraft positions and armored vehicles, while two destroyers--the USS Caron and the USS Moosebrugger--fired five-inch guns at Grenadan targets.

"We told the commander we want to do this absolutely minimizing the casualties to the civilian popu- lation," Gen. John W. Vessey Jr., chairman of the Joint Chiefs of Staff, said Wednesday. "If we went in there with bombs and the firepower that we would take to go into a normal war, we could do it very, very quickly, but we're not going to do that."

McDonald said again yesterday that the firepower was used "as surgically as we could."

McDonald provided more details on what Reagan said was a Cuban plan to turn Grenada into a "military bastion." He said a Cuban troop transport ship unloaded weapons Oct. 6 in St. George's, the Grenadan capital, and a Cuban transport plane brought a "delegation of military personnel" last Monday.

McDonald acknowledged that some of the Cuban activity was in response to the perceived likelihood of a U.S. invasion after the Pentagon revealed it had diverted an aircraft carrier and 1,800 Marines to the area. But he said the intense Cuban planning began before the Grenadan coup and before U.S. naval movements.

He said captured documents show that Cuban Col. Pedro Tortolo Comas was sent "to organize and supervise defenses of the island," McDonald said, and Cuban personnel "were ordered to improve their combat dispositions" following his arrival. Pentagon officials said the colonel was captured by U.S. forces.

McDonald also said a captured document describes a June 29 meeting between what he described as the chief of Cuba's Americas Department and a Cuban general who discussed a plan to send 341 officers and 4,000 reservists to Grenada. McDonald said the Cubans envisioned a total force of 6,800 for the island, although he did not amplify on that number.

Before the invasion, U.S. officials said they estimated there were between 400 and 600 Cubans on the island, most of them construction workers. In his prepared statement yesterday, McDonald said they were "impersonating construction workers." But he offered a more charitable view during his news conference, when he compared them to U.S. Army engineering battalions or Navy construction battalions.

McDonald also said the United States discovered 49 Soviets, 24 North Koreans and some Bulgarians and East Germans on the island. The Calivigny military barracks, where one of the last fire fights reportedly occurred yesterday, had been turned into "a terrorist training base," McDonald said.

Asked for more information, however, McDonald retreated slightly, saying only that documents suggested that Calivigny was a "potential place for training guerrillas."

Asked about using Grenada as a base for U.S. troops or ships, McDonald said the United States has no such plans "at this particular time."

"We would not rule it out," he added, "depending on what the scenarios are in the whole Central America and Latin America region ."

McDonald and other administration officials said intelligence information was deficient in some ways before the invasion. But, given the short time to plan it, there was no "intelligence failure," they said.

"You can't know everything," a White House spokesman said.

The White House spent much of the day trying to demonstrate that Americans support the invasion strongly. White House spokesman Larry Speakes read and distributed to reporters excerpts from approving telegrams.

Speakes said the White House received an "unprecedented" number 1f telephone calls and telegrams after Reagan's nationally broadcast speech Thursday night on Lebanon and Grenada policy, and said they were overwhelmingly favorable.

Staff writer David Hoffman contributed to this report.