Military abuses, an economy going from bad to worse and chronic neglect of rural areas by the government of President Ferdinand Marcos are fueling a growing communist guerrilla movement in the Philippines, according to an internal State Department assessment by U.S. diplomats.

While the growth of the New People's Army, the armed wing of the Communist Party of the Philippines, cannot challenge the Marcos government significantly at this stage, U.S. officials are concerned about its "slow but steady" growth in rural areas of the southern island of Mindanao, where "present circumstances are not encouraging, and the future is ominous," according to the report, a confidential cable written by the consular office in Cebu and sent here through the U.S. Embassy.

"On the surface it appears that peace and order in that area of Mindanao is more prevalent than at any time in the past several years," the account said. "However, . . . various local observers, civilian and military, suggest that this might only be the lull before the storm(s)." The report was dated last April and written after the author toured much of Mindanao.

The report was obtained from the Congress Task Force of the Coalition Against the Marcos Dictatorship and Counterspy Magazine, which said it had been leaked to them by State Department sources. The coalition is a Washington-based lobby of Philippine nationals who live in the United States.

A State Department official declined comment on the substance or authenticity of the report, but said, "This has got to stop. This is ridiculous. This is a serious breach of security."

The pessimistic assessment of conditions in the Philippines, where 15,636 U.S. servicemen are stationed, comes as the Reagan administration prepares to welcome Marcos to Washington next month on his first U.S. visit in 17 years. The administration is seeking to improve relations with the United States' most important ally in Southeast Asia.

At least three high-ranking administration officials, including Vice President Bush, former secretary of state Alexander M. Haig Jr. and Secretary of Defense Caspar W. Weinberger, have visited the Philippines and expressed support for Marcos. The Carter administration had been cool to him diplomatically because of alleged human rights violations and official corruption that also brought an embarrassing public rebuke to Marcos from Pope John Paul II when he visited the Philippines in 1981.

Mindanao has been the site of a battle between government forces and a Moslem secessionist rebel group since 1972. Last year the tide turned in favor of the government, largely as the result of a successful amnesty program that rewarded Moslem insurgents for turning themselves in, the U.S. report acknowledged.

But in 1981 the Army found itself threatened by the New People's Army with a hard core of 950 to 1,300 armed men and thousands of sympathizers. The New People's Army, which emphasizes the economic hardships of the peasants and the alleged lack of government concern, rather than ideology, has in some areas, notably San Vicente, "become more important than the local government," the report said.

Military lawlessness was cited as another reason for growing sympathy for the guerrilla movement. Although abuses decreased in areas where tension between the Army and rebels had diminished, they had increased in areas where the rebel army has been active, the report said. As examples of abuses, the author cited reports of indiscriminate killings of civilians by soldiers and kidnapings in which the military is believed to have been involved.

The New People's Army also reportedly is using "a low level of deliberate terrorism, particularly executions of erring officials or civilians" to instill its influence in some areas, the report said.

The largest of the southern islands in the Philippines, Mindanao has been hit hard by the world economic slump. "A general consensus among the area's business and professional people indicated that the major reason for NPA successes is the poor economy," the report said.

"No one foresees an early upturn, either, rather that the economy will get even worse before it gets better."

As a result of the downturn in the economy, the report read, "we encountered numerous reports of a reversal in the fight against malnutrition, and the apparent fact that some families are down to two meals a day, some only one."