MANILA, NOV. 20 -- The commander of American forces in the Pacific vowed today that the United States "will not back down to terrorists" and said the murders of three U.S. citizens will not deter support for President Corazon Aquino.

Adm. Ronald Hays also told a group of American businessmen that American military bases here are vital for U.S. global strategy. But he acknowledged that an antinuclear provision in the Philippine constitution might cause problems in maintaining the installations.

"The people of the Philippines have suffered in the hands of terrorists for a number of years now, and Americans have been caught up in these senseless killings," Hays said. Gunmen believed to be communist rebels killed three U.S. servicemen and a Filipino of U.S. descent during coordinated attacks on Oct. 28 outside Clark Air Base, 50 miles north of here.

Filipino police yesterday charged 19 people with murder and attempted murder in the attacks, but only one has been taken into custody.

"Those responsible for the recent murders at Clark, which may have been an attempt to drive a wedge between our two countries, will find that the crime does exactly the opposite, that it is counterproductive," Hays said. "We will not falter in our resolve to assist our friends."