The Pentagon will use "code words" in telephone conversations organizing news media pools to accompany secret military operations in an effort to prevent foreign intelligence agencies from intercepting the messages, Assistant Secretary of Defense Michael I. Burch said yesterday.

Burch said that during last month's exercise, in which a news pool accompanied a simulated invasion in Central America, news executives placed overseas telephone calls and discussed what was meant to be a secret operation.

"NBC called Tom Brokaw on an open line to Managua, Nicaragua," Burch told a forum sponsored by the Washington Journalism Review. Later he said the call went to Brokaw's producer and that he heard about it from other news sources.

Those types of calls "light the whole world up," Burch said in a reference to intelligence facilities that the United States and the Soviet Union maintain worldwide to intercept international telephone and cable traffic.

Burch cited the April 21 overseas and domestic long-distance telephone calls as a way for the Soviets to obtain advance information on such operations.

He said the Pentagon is checking its intercepted Soviet messages, apparently to see if Moscow's intelligence picked up the calls.

Burch called two news executives at the forum "naive" for saying that the Pentagon had broken secrecy by giving the first official word on the media pool in confirming a Washington Post article published April 22.

He acknowledged, however, that although the Pentagon had told news executives to limit their conversations to those reporters actually going on the exercise, the "script . . . did not say to newsmen 'Do not use the telephone.' " He said that will change.

Burch also said there were several sources for the leak of the media pool's existence -- "some on our side" and others in the media.

Bill Kovach, Washington bureau chief of The New York Times, described the Pentagon's planning of the media-pool exercise, in which The Times participated, as "amateurish."

Kovach said that because of earlier difficulties in getting Burch to talk about guidelines, he was "flying blind" during the exercise and is now suspicious that it was held "to prove . . . that the press can't be trusted."