Reshuffles in the military leadership and dissatisfaction over election fraud are causing strains in the Philippine armed forces, according to senior officers.

At the top of the military establishment, a long-simmering rivalry between chief of staff Gen. Fabian Ver and his deputy, Lt. Gen. Fidel Ramos, appears to be breaking into the open as President Ferdinand Marcos shows signs of backtracking on Ver's announced retirement.

Marcos warned obliquely Friday against congressional moves to cut off U.S. aid to his embattled government, denouncing "modern-day imperialists," and announced a diplomatic offensive to buttress his claim as the country's legitimate head of state. Details on Page A16.

The 66-year-old Ver, a cousin and close confidant of Marcos, has been a source of controversy at home and abroad. Washington has tended to view Ver as a major obstacle to military reforms seen as vital to countering a growing Communist insurgency. Despite Marcos' announcement of his retirement Sunday, Ver continued issuing orders this week, maneuvering favored officers into key positions.

In the lower ranks, disgruntlement over this month's disputed presidential election in which Marcos was officially proclaimed the winner by a parliament he controlled, surfaced publicly today with the defection to the opposition of a Philippine Constabulary captain. After reading a scathing open letter to Marcos at a press conference, the young officer announced his support for opposition presidential challenger Corazon Aquino and claimed that a majority of young officers shared his feelings.

"This is to inform you, sir, that you failed to receive the mandate of the Filipino people in the last elections," wrote Capt. Juan Vicente Resurreccion. "Every Filipino knows that except for you."

In private conversations, members of a loosely organized group of officers advocating major military reforms have expressed similar sentiments.

At the same news conference, Aquino's running mate, Salvador Laurel, asserted that "a great majority in the military are totally disillusioned with the Marcos government." He said, "I call upon all members of the military, particularly the young military officers and soldiers, to join the people in refusing to obey the immoral and illicit orders of a dying regime."

Laurel denied the opposition was "calling for a coup in the military" but said he believed many military men would refuse to obey orders to crush by force an emerging civil disobedience movement being organized by Aquino supporters.

He said the opposition was considering establishment of a "provisional government" as one of its options and that such a move "will depend a lot on military support." If the military is divided, he said, this "may result in civil war. That's why we are being very cautious."

Following the formal announcement by the Philippine government early Sunday that Marcos had been reelected in the Feb. 7 presidential poll, the 68-year-old leader said at a news conference that he had accepted Ver's resignation.

Ver went on a "leave of absence" in 1984 when he, 24 other military men and one civilian were charged with involvement in the assassination of opposition leader Benigno Aquino Jr. Marcos replaced Ver during this period with Ramos, also a distant cousin, serving as vice chief of staff and commander of the Philippine Constabulary.

When Ver and the other defendants were acquitted in December 1985, Marcos reinstated his trusted aide as chief of staff over the objections of Philippine opposition leaders and Reagan administration officials. Marcos appeared to make a concession to the United States, albeit one designed to soften condemnation of electoral fraud by his ruling party, when he announced Ver's long-awaited retirement Sunday.

"In the meantime," Marcos said, "the acting chief of staff is Lt. Gen. Ramos." Marcos also said Ver was resigning from his post as the country's overall intelligence chief in charge of the National Intelligence and Security Authority.

However, Marcos announced the next day that Ramos would officially take over as acting chief of staff on March 1, enabling Ver to "wind up his official affairs and say goodbye to the units in the field," according to a press release from the Malacanang presidential palace. Later Monday, the palace issued a "correction" stating that Ver's retirement had taken effect Sunday as originally announced.

Nevertheless, Ver appeared to ignore the announcements and continued issuing orders and assignments this week. One officer summarily relieved by Ver was Col. Reynaldo San Gabriel, the armed forces spokesman, who was brought out of retirement by Ramos last year. The move Tuesday, two days after Ver supposedly retired, left no doubt that he remained in charge.

Yesterday, Ver ordered field commanders to intensify information campaigns on the election and instruct troops that Marcos was proclaimed the winner according to law. Ver stressed that Marcos remained commander in chief of the armed forces and told field commanders that the proclamation was recognized by "all sectors except some opposition members and Namfrel," a citizens' poll-watching organization.

Ramos told reporters yesterday that he had asked Marcos to freeze all military reassignments to "prevent further jockeying and maneuvering and positioning" by some officers. He said the maneuvering was "causing apprehensions" in the military, which he said was "in a very unstable situation."

Ramos also said that he was still awaiting written orders confirming his appointment as acting chief of staff.

Military sources said Ver, backed by Marcos, apparently was moving to install loyalists in key positions to maintain effective control over the armed forces after Ramos takes over.

"Ver is trying to plug all the holes" with officers he can rely on, said a colonel formerly associated with him. "These people are all known hard-liners for the president. They will make it very uncomfortable for the reformist officers."

According to a military intelligence officer, Marcos's backtracking on Ver could be intended as "a signal to the United States" that the retirement may be rescinded if U.S. pressure builds further.

In his open letter to Marcos, Capt. Resurreccion singled out the issue of misplaced military loyalties as a major factor in his decision to repudiate publicly the 20-year-old Marcos government.

He said that in the years since he graduated from the Philippine Military Academy in 1974, "It slowly dawned on me that there is no place for a professional soldier in the AFP Armed Forces of the Philippines as long as you are president. The AFP today stinks of corruptions, politicalization, generals who are better sycophants than leaders and men whose loyalty is directed toward you and not the Filipino people."

Resurreccion, 29, a former security officer for Defense Minister Juan Ponce Enrile, said, "I am not a high-ranking general or colonel. I am but a mere soldier . . . " However, he said, "It is I who have more at stake than the generals or colonels. It is my future that is at stake. I am one of a younger generation. The generation, sir, that will pay for all this. When you are gone, you will leave us with a debt that was mainly due to the extravagance of your regime and your family."