Communist rebels battling the government of President Ferdinand Marcos appear to be running into some difficulties as they try to expand their operations, according to both military and opposition sources.

The problems stem from the efforts of the New People's Army (NPA), the armed wing of the Communist Party of the Philippines, to operate in larger formations as part of a series of "tactical offensives," the sources said. The requirements of equipping, feeding and moving these larger units of up to 400 "NPA regulars" have put strains on an organization that has negligible foreign support, they said.

While the Philippine military claims its counterinsurgency campaign has become more effective, the rebels' difficulties appear to be chiefly products of their own growth and some of their own mistakes in promoting it.

The problems also indicate that the Philippines' current economic crisis may be a double-edged sword for the insurgents: while it helps them recruit, it also dries up sources of funding needed to push the rebellion to a higher stage.

In an effort to overcome their supply problems, the rebels have begun experimenting with agricultural communes and, according to a military report, even formulated plans to produce their own weapons on the southern island of Mindanao.

While the NPA has been pursuing a strategy of fighting the Marcos government nationwide, it is on Mindanao that the greatest guerrilla war and counterinsurgency campaigns have been waged. According to a military briefing for correspondents in Manila Wednesday, 45 percent of the barangays, or precincts in which the insurgents maintain a presence, are on Mindanao. According to government figures, the NPA has up to 12,000 "regulars," two-thirds of them armed.

The military estimates that 13 percent, or more than 5,400, of the country's 41,615 barangays are either "influenced" or "infiltrated" by the Communist rebels. Western diplomats regard the terms as euphemisms for degrees of NPA control over these communities.

The acting armed forces chief of staff, Lt. Gen. Fidel Ramos, acknowledged in the briefing that rebel attacks this year had increased 15 percent from last year. But he said that military reforms and increased "abuses" by the Communists were hurting the insurgency.

According to Col. Rodolfo Biazon, commander of the 3rd Marine Brigade based near here, NPA "taxation" and several assassinations associated with it have alienated a number of communities that formerly supported the rebels.

Biazon said that 24 barangays in the province-sized area known as Davao City that were under NPA control have been "turned around" in recent months. He cited the case of Tamugan, a poor village of about 3,000 persons that formerly had contributed 15,000 pesos a month ($833) to the NPA. After a Protestant minister and two other men were executed publicly by the NPA for protesting the taxes, Biazon said, the villagers asked the Marines' help in getting rid of the rebels.

Biazon also said the NPA's 130-member regional guerrilla unit No. 5 was forced to leave the Davao City area because of increased military activity. Of the 157 barangays in Davao City, an area roughly 50 miles long by 30 miles wide with about 1 million inhabitants, the NPA formerly controlled nearly 30 percent but now hold only about 5 percent, Biazon said.

Rebel supporters and sympathizers conceded that the NPA might have suffered some "temporary setbacks" at places like Tamugan, but insisted that they were largely irrelevant at this stage of largely hit-and-run guerrilla warfare.

"It may be true that armed men of the NPA have left the place, but that does not mean the military has neutralized the place," said a rebel Roman Catholic priest. "Maybe the military has disabled the movement in that place, but how long can it hold the sympathy of the people?"

The priest, who did not want to be identified but acknowledged his affiliation with the rebel movement, also conceded that "there may be instances in certain localities of abuses or mistakes on the side of the NPA." But he said people sometimes may be misled because "several groups operate under the name of the NPA."

With increased military activity in the area, the priest said, "It is more difficult for the movement to operate here." He said he knew of several cases in which mounting hardships had driven "NPA partisans" to the government side and said that such informers had "done quite a lot of harm" lately. In the past three months, he said, about 100 NPA suspects fingered by informers had been brought to the Philippine Constabulary prison in Davao City, and 40 subsequently were detained, the priest said.

However, neither he nor any other opposition or military sources knew of any mass surrenders that would buttress President Marcos' recent assertion that the insurgents now "are starting to surrender in droves."

The government's claimed successes also apparently have not resulted in any decrease in violence in Davao City, the Philippines' "murder capital." In fact, the number of killings here -- most but not all of which are attributed to the military or the NPA -- exceed 530 so far this year, compared with about 450 in all of 1984.

In addition, despite the proclaimed military reforms and the generally good reputation enjoyed by Col. Biazon, human rights activists charge that military abuses continue. According to both opposition and government sources here, summary executions known as "salvagings" are on the rise, beatings and torture of political detainees are practically routine, looting by military men during "zoning," or dragnet operations in suspected Communist-infiltrated neighborhoods is a source of continuing public resentment, and much military intelligence activity is wasted on information-gathering for the purposes of extortion.

Meanwhile, the rebels quietly are working on the problems associated with escalating their armed struggle, the sources said.

"They're finding difficulties in logistics," said a knowledgeable opposition lawyer. "They have not yet mastered that higher level of warfare. For example, they have not yet solved the problem of feeding a big number of people."

In what he described as "a very sophisticated move," the lawyer said the rebels were even experimenting with legitimate agricultural ventures by entering into contracts with landlords as tenant farmers. He said the rebels were willing to provide all the capital and labor and give the landlord 30 or 40 percent of the farm's earnings as rent.

"It's a cover-up for a commune system," the lawyer said. By contracting with landlords instead of trying to take over remote farm lands outright, he said, rebel supporters could claim legal status for their communes.

In a recent interview, a Communist Party cadre here acknowledged that "we are organizing certain farmers into communes," but he declined to give further details.

According to Brig. Gen. Jaime Echeverria, who heads the military's Regional Unified Command here, the rebels also plan to organize local gunsmiths into arms-producing ventures. Echeverria said the military had captured Communist documents showing that the rebels were dissatisfied with their agaw armas, or "arms grabbing," campaign of seizing weapons from the armed forces.