Administration officials, while publicly accusing Nicaragua of invading Honduras with 1,500 troops, privately acknowledged yesterday that they are uncertain about the size of the Nicaraguan force and about other details of the headlined conflict in Central America.

Few in official Washington are questioning that some Nicaraguan soldiers crossed into Honduras to fight the antigovernment rebels based along that border, but nearly everything else seems less certain than formal government statements here suggested.

"The first intelligence reports showed 1,100 Sandinistas crossing the border," said a senior administration official. "Subsequent reports then ranged from 500 to 1,500 . . . . It was getting very soft."

Questions about the size of the incursion were accompanied by uncertainty about the sources and reliability of U.S. information, the attitude of the Honduran government and the reasons why the administration chose to treat it as a major event.

A Defense Department intelligence report Tuesday put the number of Nicaraguans fighting in Honduras at 800. However, administration officials told House members in a closed briefing that day that 2,200 troops had crossed the border, according to Rep. Peter H. Kostmayer (D-Pa.). The White House has used the number 1,500 in public statements.

Sen. David F. Durenberger (R-Minn.), chairman of the Select Committee on Intelligence, said he was told Monday that "two large groups" of Nicaraguan troops numbering 800 to 850 each had crossed into Honduras on Saturday to attack a base camp of the counterrevolutionaries, or contras, while an additional 300 Nicaraguan troops had taken over a contra airfield near the border -- for a total of 1,900 to 2,000 troops. Durenberger said the information was "basically coming from the contras, but it has CIA confirmation as well."

A similar incursion of 1,000 or more Nicaraguan troops in the same general area took place last April, according to U.S. intelligence officials. But little or nothing was said publicly at the time. Defense Secretary Caspar W. Weinberger said the current "invasion" is one of the Sandinistas' largest and the deepest into Honduras.

Intelligence received in Washington last weekend about border clashes left it unclear until Monday morning that the conflict was more serious than hundreds of previous small-scale engagements, according to a senior State Department official.

On the other hand, President Jose Azcona of Honduras said in Tegucigalpa last Friday that he had established contact with Nicaraguan President Daniel Ortega because of "the tense situation that has emerged on the border," according to a news dispatch that night.

Another news report from Tegucigalpa around 1 p.m. last Saturday said several Honduran army units had been placed on "red alert" in view of "strong Sandinista contingents" deployed at the border.

A State Department official said the Honduran government initially "had no idea" what was happening in its mountainous border area. "The U.S. intelligence was probably better than theirs" and was being shared with Honduran officials, this official said.

Around 10 a.m. Monday, according to the official, Azcona telephoned Washington to say his government was faced with a "very massive" incursion and "probably would need at least helicopter lift support." A few minutes later, the official said, another call came, this one from Gen. Humberto Regalado Hernandez, commander of Honduras' armed forces.

Azcona's call was made to Assistant Secretary of State Elliott Abrams, who made an unannounced trip to Tegucigalpa last Friday. Officials said the trip was to reassure the Honduran government following Thursday's House vote against President Reagan's $100 million contra aid program.

Azcona spoke Monday morning to Abrams' deputy for Central America, William G. Walker, because Abrams was at the White House. A senior U.S. official, identified in news reports as Abrams, told White House reporters about 10:30 a.m. Monday of "a report today of a very large Sandinista incursion into Honduras." Reporters were told that "well over a thousand Nicaraguan troops" were involved, according to "the initial report," but cautioned that an assessment was still under way.

As late as 11:15 a.m. Monday, Azcona's press secretary was denying in Tegucigalpa that there had been a Nicaraguan incursion. The secretary described such reports as "confusion" and "disinformation."

Early Monday afternoon, Azcona called back to say he was making a formal request for U.S. assistance, according to State Department sources. This time he spoke to Abrams, who was quoted as saying that the administration would begin deliberations on the basis of the phone call but that a written request would be needed.

Abrams was also quoted as telling Azcona that "we'll hope you'll back us up and admit to the world what is going on." Until that point, the Honduran government was saying nothing publicly, following a policy of silence apparently intended to avoid admitting that contras are based in that country.

In a statement Tuesday, Azcona announced that Nicaraguan troops had violated Honduran territory and that he had asked for U.S. help. By then he had sent a formal request to President Reagan for "air transport of Honduran troops as necessary and other material assistance that may be necessary to repel the Sandinista forces and prevent these attacks from recurring."