A Philippine Moslem guerrilla faction has warned the United States that the government of President Ferdinand Marcos faces renewed rebellion in the fall unless it lives up to 1976 agreements for more autonomy and economic rights for the country's Moslem population.

The renewed demands come at a time when the Philippine armed forces are already under severe strain fighting a growing Communist insurgency, particularly on the southern island of Mindanao. Most of the Philippines' 2.5 million Moslems live on Mindanao and the neighboring Sulu archipelago. Moslems make up about 5 percent of the mostly Catholic population.

Dimas Pundato, chairman of the executive council of the Moro National Liberation Front, said his forces would "take up the armed struggle" in November if a political settlement could not be reached.

Pundato and the front's international spokesman, Macapanton Abbas, met last week with State Department, Pentagon and National Security Council officials. U.S. officials declined to elaborate on the discussions, but a State Department official said: "We maintain an open-door policy. We will talk to anyone with a responsible point of view." A source close to the Moros said their aim is "to have the United States use its moral authority and put pressure on the Philippine government" to meet their demands.

Although Pundato says there are many ideological clashes between the Moro front and the Communists, he acknowledged that local commanders have cooperated with Communist guerrillas. But if an agreement can be reached, Pundato said his Moslem forces could help Manila fight the Communists.

The Moro insurgency reached its height in the mid-1970s when the rebels engaged up to 70 percent of the Philippine armed forces' combat battalions, some analysts say. Some observers in the Philippines and the United States have said the Philippine military was so tied up with fighting the Moslems at that time that they paid little attention to the Communists. Now, a decade later, the phenomenon may be reversed, these observers say.

For the most part, the Moslem insurgency died down after a ceasefire and the 1976 agreement negotiated in Tripoli with the Marcos government. Many of the major field commanders have since surrendered and the leadership of the front, which is backed by Saudi Arabia and other moderate Islamic states, subsequently has split into at least three major factions.

Pundato, whose group is believed to be among the more moderate factions, says he has between 5,000 and 6,000 armed rebels under his command, with 12,000 in reserve.

Under the 1976 accord, Marcos agreed to create an autonomous region out of 13 Moro provinces with its own Moslem-led government, an integrated peace-keeping force and a freely elected local assembly that would recognize traditional Moslem laws.

Philippine Embassy officials here say they have "closed the case" on secession and that Pundato does not speak for the majority of Moslems.

[In another development Saturday, Marcos said he would try to settle without "emotion" whether to extend the treaty covering U.S. military bases in the Philippines, United Press International reported from Manila. "This is too critical an issue to be decided with emotion," Marcos said.]