A key member of the Philippines negotiating team said today that talks on control of U.S. bases here remain hung up by differences over Manila's right to try lawbreaking Americans and uncertainty over U.S. willingness to defend the islands from attack.

Former vice president and foreign secretary Emmanuel Pelaez, who has helped negotiate U.S. base agreements for more than 20 years, made perhaps the frankest Filipino public statement yet on the fear of war over South China Sea islands - probably rich in oil - claimed by the Philippines, Vietnam and China.

"Let's say the Philippines was attacked in Palawan," an island where Philippines prospecting is under way, said Pelaez. Under the current mutual security treaty "you can respond only in accordance with constitutional processes, which in your case means Congress. In the meantime your President cannot commit any portion of his forces to repulse this aggressor."

Actually, in the ongoing debate in Washington about the President's prerogatives under the War Power Act - passes despite a presidential veto in 1973 at the height of reaction to the Vietnam war - there is little question that he can send aid and even troops into action under an existing treaty before obtaining the consent of Congress.

The Philippines would like a U.S. promise to rush military supplies or other help short of troops, said Pelaez. He quoted President Ferdinand E. Marcos, who has taken an unusually active role in recent discussions, as saying, "Let's look at possible contingencies that could give us a capability to respond until such time that your Congress has made its decision."

Pelaez said the recently negotiated Panama Canal treaties and congressional reaction to their waiving of longheld U.S. authority in the Canal Zone could have a decided impact on efforts here to increase Filipino control over U.S. bases. He recalled that in 1956, U.S. diplomats were ready to let the Philippines flag go up over the bases but pulled back when the Pentagon began to worry "about what the repercussions would be Panama."

Palaez expressed confidence that the two sides would eventually reach a new agreement over U.S. use of the huge Clark Air Base and Subic Bay Naval Base, the last major U.S. military installations in Southeast Asia. "I hope we get an agreement within the next year, but we wonder if the Carter administration can give this enough attention, when it has the energy bill nad the Panama Canal Treaty," Pelaez said.

In the wake of the U.S. withdrawal from Vietnam in 1975 and Marcos' decision to move this former U.S. colony toward a nonaligned foreign policy, talks requested by the Philippines over renegotiation of the existing bases agreement began in mid-1976.

The Philippines won assurances that Filipino commanders would assume titular control over the bases and seems sure to win an increase in compensation to about $200 million a year, about twice what it is paid now. But it also has asked for more conrol over actual U.S. military operations at the two bases and for the right to try U.S. personnel in Philippine courts, an explosive issue both here and in the U.S. Congress.

Pelaez, in an interview, provided an unusual amount of detail on the negotiations, whose participants have been discouraged from speaking on the record.

Pelaez, like other sources here close to the talks, said the issue of criminal jurisdiction over U.S. servicemen was proving particularly difficult to solve.

He said, "The bone of contention is this determination of duty status" - the present U.S. right to save any serviceman from Philippine prosecution simply by declaring he was on official duty when the alleged criminal act was committed. One study indicates there are about 25 such cases each year. They arouse great anger here, particularly in the few instances in which U.S. sentries have killed or wounded Filipinos.

American servicemen have been equally vehement about the need for protection against what they see as capricious Philippine justice, particularly when charges arising from arguments in bars around the bases are lodged in local courts.

Some sources here insist that the amount of compensation for the bases is still at issue, despite a tentative agreement, set aside a year ago, to pay Manila about $1 billion over a five-year period for use of the bases. Marcos must finance a war against Moslem rebels in the south and meet mounting payments on loans floated abroad, these sources say, so pressure for more money continues.

U.S. military officials apparently still object to calling the compensation "rent," as the Philippines would prefer, rather than military and economic aid that would not be so closely tied to the U.S. right to continue to use the bases.

The American negotiating panel is led by newly arrived Ambassador David Newson and the Philippines panel by Secretary of National Defense Juan Ponce Enrile, but the panels have yet to meet formally. Instead, there have been a series of informal meetings by Newson with Philippines officials, including Marcos.

Sources say this approach has brought progress, but express great skepticism at earlier reports that an agreement might be reached before the end of this month.