Reagan administration officials said yesterday that a Nicaraguan battalion of about 300 troops remained pinned down inside Honduras, and a senior official said this was "an advertisement" of the aggressive military intentions of Nicaragua on the eve of a Senate vote on a $100 million aid package for the anti-Sandinista rebels.

White House spokesman Larry Speakes said that "contrary to statements made by Nicaraguan emissaries in several locations, it now appears that as many as 300 Sandinista troops are blocked in Honduras and unable to extricate themselves."

While U.S. officials denied that the United States or Honduras was responsible for the military plight of the apparently trapped Sandinista forces, they readily acknowledged that their continued presence in Honduras was of political benefit to the Reagan administration.

"It doesn't hurt," said Dennis Thomas, the White House official who has been coordinating legislative strategy on the aid package. "It doesn't open the floodgates in the Senate , but it certainly reinforces our position."

Administration efforts to extract some advantage from the Sandinista incursion came on a day when the mood at the White House changed from optimism to uncertainty about the prospects of the contra aid package. It also produced recriminations at White House briefings, where a senior administration official accused reporters of omitting facts from their stories in a manner that favors the Sandinista government.

Speaking on condition that he not be identified by name, the senior official said he could "not understand why in this briefing room that you are so defensive of the Sandinista government there, time and time again." The official complained that many news organizations had not reported that the Honduran government, after two days of refusing to confirm the reports, had accused Nicaragua of a large-scale "incursion" in a statement issued Tuesday afternoon by its embassy here.

The Washington Post reported the statement in the second paragraph of its story yesterday morning, and it also was widely reported in wire service and other newspaper accounts. Later, United Press International reporter Ira Allen asked Speakes, who regularly conducts the briefings, why journalists' raising of questions about apparent contradictions in administration statements "paints us as pro-Sandinista."

"I just simply pointed out the fact that . . . this statement talking about the seriousness of the situation was not covered very well," Speakes responded. "And that's a fact. State the facts. Don't draw conclusions."

Speakes also objected to an NBC News report quoting Pentagon sources that said he had exaggerated the size of the Nicaraguan military operation. Asked to justify his statements, Speakes said he had told reporters "the facts" and that whatever the Pentagon sources had said were "not the facts."

In an interview yesterday, White House chief of staff Donald T. Regan minimized the U.S. tactical role in the fighting, saying the administration was "not calling the shots in that area" and adding that if the Sandinistas were caught there, "that's strictly between the two sides."

Regan said that it "would not be feasible" for the contras to pin the Sandinistas down for any length of time.

"It's not up to us to decide," he said. "We have no people in the area. We have no business in the fighting and, apart from that . . . it's somebody else's war."