MANILA, SEPT. 20 -- The Philippines, unveiling its position on the future of U.S. military bases here in talks with American negotiators, called today for full "sovereign control" over Clark Air Base "as soon as possible" and warned that all six U.S. bases in the country must be dismantled by September 1991 unless an accord is reached by next January.

In Washington, meanwhile, the State Department warned Americans in the Philippines of "a possible imminent terrorist bombing" by the communist rebel New People's Army directed against U.S. government or other public facilities in Manila. It cited a "specific and credible" threat and urged Americans to avoid the Roxas Boulevard area, where the U.S. Embassy and the Philippine Central Bank building, site of the bases talks, are located.

A U.S. official said the warning was based on intelligence information but declined to elaborate.

After a second full day of talks between the two sides, a spokesman for the Philippine delegation, Rafael M. Alunan III, said in a brief written statement: "The Philippines took the position that it shall assume sovereign control of Clark as soon as possible." In answer to questions, he said the Philippines wants "full operational control" of the huge U.S. air base 50 miles north of Manila, but he declined to say when.

If the two sides fail to agree on a treaty covering the bases, Alunan said, "then everything reverts to the Philippines" on Sept. 16, 1991, when a current agreement expires. He said the fate of Subic Bay Naval Base and four smaller facilities would be addressed Friday before the talks adjourn.

"It's not something we wanted to hear, but it's part of the negotiating process," a U.S. delegation member said of the Philippine position. He said he could not see the United States turning over control of U.S. Air Force operations to a foreign government.

The Philippine negotiating team, headed by Foreign Secretary Raul Manglapus, appears so far to be following the outlines of a draft "treaty of friendship and cooperation" it has drawn up to govern U.S.-Philippine relations in a number of areas, including the bases. The draft treaty, a copy of which was seen by The Washington Post, calls for U.S. withdrawal from Clark by Sept. 16, 1991, and from Subic by Sept. 16, 1994.

Diplomatic sources said this proposal probably would be no more than an opening bargaining position, since Philippine plans to convert the bases to commercial uses, including an airport at Clark and a "maritime industrial complex" at Subic, would take several years.

The U.S. side seeks a "phase-down" of the bases over about 10 years, followed by some form of continued access to the facilities, possibly on a commercial basis.

In a briefing today, U.S. spokesman Stanley Schrager defined the American objective as "a new security accord which includes a phased reduction in U.S. military presence and preservation of adequate operational flexibility." He said that in addition to continued access to the bases when the new accord expires, Washington would want to keep a "U.S. military presence" appropriate to both countries' needs.

But the idea of any U.S. force, however small, remaining in the Philippines well into the 21st century has alarmed nationalists here. They point out that the country's 1987 constitution calls for the removal of all foreign troops and bases by the September 1991 expiration of the current U.S.-Philippine accord, unless a treaty or a referendum extends their stay. Any treaty must be ratified by two-thirds of the 23-member Senate, where anti-bases sentiment is strong.

According to a U.S. official, Washington opposes separate expiration dates for agreements on Clark and Subic and wants any new undertaking to be an "executive agreement" rather than a treaty requiring U.S. Senate ratification. The American side also wants to give the Philippines less aid in the future as "compensation" for use of the bases than the current package of $481 million a year for fiscal years 1990 and 1991. The Philippines complained bitterly after Congress cut $96 million from this year's appropriation.

The Philippine draft treaty calls for the United States to pay $600 million a year, plus unspecified U.S. credits, for the three years until Subic would be vacated in 1994.

The draft says the bases cannot be used for "military combat operations without the prior consent of the Philippines," bars the storage of chemical, biological or nuclear weapons in the country and prohibits the passage through Philippine waters or airspace of any aircraft or naval vessels carrying such weapons. It also forbids the entry of U.S. nuclear-powered vessels and submarines into Philippine waters "without prior approval."

Similar antinuclear measures introduced by New Zealand several years ago resulted in a feud with the United States, which refuses as a matter of policy to confirm or deny the presence of nuclear arms on its ships or planes.