President Reagan said yesterday that the United States will not withdraw military officers from the U.N. peacekeeping force in Lebanon despite the kidnaping of Marine Lt. Col. William Richard Higgins and threats of terrorism.

"We are going to meet our obligations to the United Nations," Reagan said during a photo session in the Oval Office as he greeted West German Chancellor Helmut Kohl. "You know I'm not supposed to be taking questions here, but on this particular subject I feel that I must straighten it out."

The president continued, "Of course we worry because we know terrorists throughout the world targeted us as one of their targets."

The Pentagon acknowledged late yesterday that Higgins was a junior military aide to then-Secretary of Defense Caspar W. Weinberger from June 1985 until last June, when he was assigned to the U.N. observer group in Lebanon, a job acquaintances said he aggressively sought.

Pentagon spokesman Dan Howard said that as an aide to Weinberger, Higgins worked with a variety of classified documents but was only "one of 36 individuals who worked in the immediate office of the secretary handling administrative work."

Higgins, 43, of Woodbridge, was chief of the 75-member U.N. observer group, when kidnaped Wednesday morning in southern Lebanon.

Pentagon officials issued the statement about Higgins' previous job, Howard said, because Radio Free Lebanon had broadcast a report in Arabic quoting unnamed Amal Shiite Moslems as saying Higgins had an "association with Weinberger."

Pentagon officials had asked news organizations to play down Higgins' background out of concern the information could possibly jeopardize his return.

Howard said, however, that U.S. officials do not believe the kidnaping is related to Higgins' past assignment with Weinberger.

Howard also said Higgins would not have been barred from taking the U.N. assignment after leaving the defense secretary's office.

"There was no restriction that was broken by his accepting this assignment," Howard said.

Defense Secretary Frank C. Carlucci said yesterday, "We are doing everything possible" to locate Higgins.

He added, "A lot of us in this building {the Pentagon}, myself included, know Col. Higgins, and we certainly want to do everything possible to get him out."

Carlucci said that although the search for Higgins is primarily a "U.N. responsbility, not a U.S. government responsibility. . . we are concerned about our people and we'll work with the U.N. on it."

Carlucci also said the United Nations "has fanned its forces throughout the area looking for him."

Higgins, a decorated veteran of the Vietnam war as an infantry officer, was serving a one-year tour on the Lebanon observer group. Named chief of the unit in January, he was to serve as chief until his tour ended in June.

Pentagon officials said Higgins' wife, Marine Maj. Robin Higgins, who works for the Defense Department public information office, had received no news of any search results yesterday.

{In Delray Beach, Fla., Robin Higgins' parents, Norman and Rhoda Ross, expressed deep pessimism about their son-in-law's situation, reported the Fort Lauderdale News & Sun Sentinel.

{"It's been difficult to handle this," Norman Ross said. "It's taken away peace of mind."

{"It's very, very distressful," said Rhoda Ross, 50, a retired teacher.

{Their daughter and Higgins met in officer training school in Quantico, and were married at a military wedding, the Rosses said.

{When Higgins went to Lebanon last June, "It was the first time in a long time" they had been apart on separate assignments, Rhoda Ross said.}