MANILA, NOV. 9 -- The Philippine government, showing signs of an internal rift over negotiations with the United States on military bases, today appeared to back away from an earlier vow to assume full control over a huge U.S. air base by next September.

After concluding a second round of talks on the future of six U.S. military bases here, spokesmen for the American and Philippine negotiating teams agreed that "progress" had been made in forging an accord to replace the 1947 Military Bases Agreement between the two countries. However, neither official would identify any area of agreement.

Philippine spokesman Rafael M. Alunan III said at a news conference that "the Philippine panel is standing by its framework of assuming sovereign control of the military facilities by next year." But in response to questions about the government's announced intention of taking over Clark Air Base by Sept. 16, 1991, when the current bases agreement expires, Alunan differentiated for the first time between "sovereign control" and "operational control" of the sprawling base 50 miles north of Manila.

He suggested that Philippine military and civilian personnel would not be fully ready to take over the base by then and that Americans would be needed to operate it -- under Philippine sovereignty -- beyond the expiration date. During an opening round of talks in September, the Philippine side said it wanted "sovereign control" over Clark, which Alunan defined as "full operational control." The concept alarmed U.S. negotiators, who argue that the United States cannot cede control over operations of its Air Force.

{On Saturday, unidentified assailants fired two grenades at the U.S. Embassy, the Associated press reported. There were no injuries in the attack, which took place at 6:50 a.m. when the embassy was closed.}

Despite positive public statements by both sides today, sources close to the talks said the U.S. panel was beginning to feel some frustration over the slow pace of the negotiations and the Philippine side's failure so far to respond specifically to U.S. proposals.

The American team, led by special negotiator Richard L. Armitage, has proposed a gradual reduction of the nearly century-old American military presence here over 10 to 12 years, followed by continued U.S. access to the bases.

Alunan said today that if the two sides have not reached an agreement by Jan. 31, "it will be necessary to activate a mechanism that will oversee a dismantling of the facilities." .A new round of talks is planned for next month.

Alunan also said the Philippine negotiators welcomed a U.S. announcement Wednesday that all fighter planes based at Clark would be removed by next September, along with about 1,800 American military personnel.

The move, which reportedly will cost a few hundred Philippine workers their jobs at Clark, has rattled the adjacent community of Angeles, which generally supports the U.S. presence on economic grounds. Nearly 80,000 Filipinos are employed at Clark and Subic Bay Naval Base, which are estimated to pump about $1 billion a year into the Philippine economy.

The vice chairman of the Philippine negotiating team, Health Secretary Alfredo Bengzon, an opponent of the bases, raised a furor earlier this week by complaining that the panel was coming under "pressure" from government economic managers and the Philippine military brass to agree to retain the U.S. bases. Bengzon identified the officials as Finance Secretary Jesus Estanislao, Central Bank Governor Jose Cuisia Jr. and the armed forces chief of staff, Gen. Renato de Villa, and suggested that their opinions reflected the view of Washington.

Bengzon said the economic managers feared that getting rid of the bases would badly cripple the Philippine economy, which already is staggering under heavily increasing oil costs, and would adversely affect debt-reduction efforts and negotiations with the International Monetary Fund. Officials said Bengzon's remarks revealed a split within the government, and senators pressed for an investigation of his claims.