Reagan administration officials described the latest coup attempt in the Philippines as the most serious challenge to President Corazon Aquino since she took power 18 months ago, and the administration issued unusually strong statements condemning the insurrection and supporting Aquino.

A statement of "profound concern" in the name of President Reagan issued by the White House early yesterday morning was designed to persuade all elements of the Philippine military of Washington's "unqualified support" for Aquino, U.S. officials said. A senior State Department official said there were indications that the Reagan message, which was authorized by the president after telephone conversations with national security adviser Frank C. Carlucci, had positive results in Manila.

State and Defense Department officials said there had been many reports of dissatisfaction in some elements of the Philippine military with the civilian and military policies of the Aquino government, but that no advance warning of the coup attempt had been picked up by U.S. intelligence agencies.

"Nobody knew anything about the specifics of this," said a Pentagon official. "If we had known, we would have alerted {Philippine armed forces chief of staff Fidel V.} Ramos." The official attributed the intelligence failure to the absence of a confidential, trusting relationship between the United States and pro-Marcos or other Philippine Army dissidents.

Early reports reaching Washington on Thursday afternoon suggested that the insurrection had been put down quickly, but by mid-evening officials had become concerned by news that the revolt had not been quelled and especially by reports that military commands in Cebu and other areas of the far-flung island nation were joining the rebellion or in doubt. By midday yesterday, Washington time, the tide of reports through embassy and intelligence channels and the news media seemed to have turned again with indications that nearly all the military high command in the Philippines was loyal to the Aquino government, which was progressively reasserting its control.

Officials said there were no indications that deposed president Ferdinand Marcos was involved in fomenting the rebellion, although during the early hours Ramos publicly identified the rebels as followers of Marcos. The U.S. administration, apprehensive that Marcos might be involved and uncertain of his whereabouts, sent officials to his exile home in Honolulu shortly after the revolt broke out to make sure he was there.

Marcos, appearing on ABC's "Nightline" program early Friday morning, said that Aquino was "in real trouble" and that he would be willing to return to power if rebel officers overthrew her. Marcos denied he was in touch with the rebels or was financing them.

The current insurrection was considered more serious than previous coup attempts because it seemed to arise not from Marcos or his partisans but from the adherents of the more broadly respected Reform the Armed Forces Movement (RAM), which had opposed Marcos in his final months and had demanded changes.

Following the overthrow of Marcos and the installation of Aquino, according to Washington officials, RAM leaders in Manila were dispersed around the Philippines by Ramos so that there would be no natural center of potential dissidence based on RAM allegiances. The recollection of this dispersal worried U.S. officials late Thursday when they heard reports that the rebellion was winning support in diverse areas.

Washington officials said they considered it particularly important that Aquino appeared to be taking a very tough personal stand against the rebels, referring to them in English as "traitors" and in Tagalog as "bastards."

Previously she and the military high command have taken a relatively lenient stand against plotters.

United Press International reported that U.S. military bases in the Philippines have been "buttoned up" and U.S. military personnel cautioned not to become involved in any way with the coup plotting.

The United States has 16,290 military personnel in the Philippines, mainly at Subic Bay naval base and Clark Air Base.Staff writers Jay Mathews in Los Angeles and David B. Ottaway in Washington contributed to this report.