MANILA, Philippines (AP) _ The United States today offered to reduce its troop strength in the Philippines but expressed hope it could maintain a military presence ''which both countries would judge appropriate.''

U.S. negotiators made the offer during the third day of talks on the future of Clark Air Base, Subic Bay naval base and four smaller installations, whose lease expires in September 1991.

Philippine negotiators said they wanted to take operational control of Clark as soon as possible, although the proposed timetable was unclear. Philippine spokesman Rafael Alunan said the government was waiting for a U.S. counterproposal.

U.S. spokesman Stanley Schrager told reporters that the American team, led by chief negotiator Richard Armitage, outlined its proposals for a military presence after 1991.

''We would hope that the relationship would continue with appropriate U.S. access to and use of the Philippine facilities, together with a U.S. military presence, which both countries would judge appropriate to meet our needs,'' Schrager said.

The discussions were to continue Friday. Alunan said the talks were expected to be adjourned then to an undetermined date.

Earlier today, Alunan said President Corazon Aquino would order the dismantling of the six U.S. military bases unless U.S.-Philippine talks produce an agreement by January.

''If there is no treaty or arrangement, let's say by January, ... then we will most likely create a committee that will oversee the dismantling of the bases,'' Alunan said.

''It is important for us to have a treaty or an arrangement,'' he said in a radio interview. ''If there is no understanding, nothing will come of the bases such that by Sept. 16, 1991, they will have to leave.''

The United States maintains 40,000 troops, Defense Department civilians and military dependents at the six bases.

The bases are technically under Philippine control but are effectively run by the Americans, who ruled this nation from 1898 until independence in 1946.

Mrs. Aquino said on the eve of the talks that the time had come for an orderly withdrawal of the bases, the oldest and largest American overseas installations.

Any treaty to keep the bases after the lease runs out requires approval by two-thirds of the 23-member Senate, where opposition is strong. The Philippine Congress could also require the extension to be approved in a national referendum.

Opponents, such as Communist rebels, believe the bases infringe on national sovereignty.

In Washington, the State Department warned Americans in the Philippines of ''a possible imminent terrorist bombing'' by the communist New People's Army against U.S. government or other public facilities in Manila.

A brief statement attributed to spokeswoman Margaret Tutwiler said the danger was greatest ''in the vicinity of Roxas Boulevard,'' where the U.S. Embassy is located and where talks on the future of the six U.S. military bases in the Philippines were being held.

American civilians said they were contacted by the embassy today and advised of the alleged threat.

In a related development, the Aquino government handed out a 10-page pamphlet today to the news media that sought to minimize the bases' political and economic importance.

The government primer on the bases says the bases are not needed to prevent the overthrow of the government. In December, U.S. jets from Clark flew air cover for loyal troops during a coup attempt that claimed more than 100 lives. Later, the government belittled the American role in saving Mrs. Aquino.

The bases' contribution to the economy in 1990 will amount to ''only 1.9 percent'' of the gross national product, the pamphlet says.

The bases have created jobs for 130,000 Filipinos.

The facilities generate child prostitution, AIDS and other sexually transmitted diseases, abandoned Amerasian children, drug addition and dislocation of tribespeople from their ancestral lands, according to the pamphlet.

But the pamphlet says that even if the bases are closed, the Philippines will still be ''committed to continue the friendship the country has had with the U.S. for decades.''