The United States is continuing to monitor military communications on the Chinese mainland from a closely guarded base here despite the new Peking-Washington relationship.

The existence of the facility in the hills north of Taipei is such a sensitive issue that a U.S. military spokesman first denied that there was such a base.

All official Americans, including the 700 remaining U.S. military personnel and their 900 dependents, are supposed to leave Taiwan by April 30 as part of the normalization of relations with Peking. The U.S. military spokesman here declined to say whether or not the Americans manning the listening post, named Shu Lin Kou, would be among them.

Under legislation now before the Congress, U.S. interests on Taiwan are to be represented by a federally funded private organization, the American Institute on Taiwan, as of March 1. Amercian civilians working for the institute would leave government service.

State Department and Pentagon officials declined to comment on the future of the base Friday other than to say, "Under current guidelines, no U.S. personnel are to be permanently stationed on Taiwan" after April 30.

[They declined to comment on whether the base could be turned over completely to Taiwan or whether it could be operated in some other fashion, perhaps by Americans from private companies under contract.]

Under the normalization agreement with Peking, the United States says relations between Taiwan and the mainland are an "internal" Chinese matter, recognizing Peking's legal, if not its actual control over the island.

Continued operation of a U.S.-linked intelligence base, even in an unofficial capacity, would appear to be against the spirit of the normalization of relations with Peking. As one observer noted, the United States continues to monitor Soviet military communications, but not from territory it recognizes as being part of the Soviet state.

Chinese officials in Peking are believed to be aware of the base and its U.S. ties and are understood to expect Washington to handle the question of the base's future quietly.

Although the U.S. military spokesman declined to say whether or not the U.S. role at the base is included among the activities to be ended by April 30, a Taiwan government official said that negotiations are under way to allow the United States to keep the base. The U.S. military spokesman and the U.S. Embassy spokesman here both denied any knowledge of such talks.

When first asked about the existence of the base Thursday, the U.S. military spokesman here replied that there had been one but that it was closed "over a year ago."

To a visitor to the base today however it was clear that although the American name and the U.S. flag had been removed, American personnel in civilian clothes were very much in evidence. They were driving military vehicles and civilian cars with military license plates.

Asked again about the base and the presence of Americans there, the spokesman today conferred with higher authorities and later read a formal statement in the name of the U.S. Taiwan Defense Command -- the highest U.S. military authority on the island, headed by Rear Ad. James Linder. The statement says: "Using Department of Defense contract personnel, IDC (Taiwan Defense Command) operates a communications system at Shu Lin Kou in mutual support of the ROC (Republic of China) and associated U.S. activities

He would not say what sort of contract the personnel were fulfilling for the Pentagon nor how many were at the base. Taiwanese farmers around the perimeter of the sprawling base said they had seen 10 or 20 Americans there. An informed source said the contract personnel are employees of the National Security Agency -- the super-secret organization responsible for all communications intelligence.

According to the source, the base was set up during the height of the Cold War as part of an intelligence gathering ring around China which consisted of bases in Japan, Okinawa, Taiwan, the Philippines, Vietnam and Thailand. Only the first four remain active today.

The base reportedly originally housed hundreds of U.S. Air Force, Army and Navy personnel, each group concentrating on the communications of its Chinese counterpart. A few years ago, as the United States began reducing its military presence here from a high of 10,000 to the current level of 700. U.S. military personnel operating the monitors reportedly were replaced by Taiwanese troops.

At the base gate, now guarded by helmeted Taiwanese guards carrying rifles with fixed bayonets, Americans entered and left freely in their military cars.

I was stopped at the gate. After explaining that I wanted to talk with the American in charge, I was put in phone contact with an American secretary who identified herself as "Mrs. Reed." She said I had to wait to be escorted inside.

A few minutes later, a Maj. Chang called the guardhouse and said that there were no Americans there and that I should leave. Asked for the name of the American in charge, Maj. Chang paused and a male with a southern drawl could be heard faintly but clearly in the background: "Tell him you don't know my name." Maj Chang then said he did not know the name.