Defense Secretary Frank C. Carlucci, after three hours in his new role, said yesterday he will bring "an entirely different approach" to some parts of the job, but told his senior staff there would be "no purges in the Pentagon."

Carlucci also said he would cut U.S. naval presence in the Persian Gulf only "if we can reduce the nature of the threat."

Carlucci, who was welcomed to the Pentagon with an honor guard ceremony in the inner courtyard yesterday morning, said the Joint Chiefs of Staff are working on ways to cut the force "as the nature of the threat changes.

"If you reach a steady state, there might be changes that you can make," Carlucci said. "I don't know if we've reached that steady state."

On other issues, Carlucci said he will attempt to smooth relations between Congress and the Defense Department, which had deteriorated under the inflexible budgetary arguments of his predecessor, Caspar W. Weinberger.

"I have an entirely different approach to say how you negotiate with the Congress," Carlucci said. "I will defend that budget just like Cap Weinberger did . . . . Once it becomes clear that the Congress has a diffent set of priorities or if they are going to cut that budget . . . then I would be prepared to work quietly with them to do that."

The new defense secretary also said he his prepared to recommend cutting some weapons programs and some military forces to meet future budget constraints.

"Somebody's gonna have to feel the pain," he said. "They're going to have to be some cutbacks somewhere."

And although Carlucci said he will continue to push hard for support of the president's Strategic Defense Initiative, known as "Star Wars," he added, "It's like anything else in the Pentagon budget. It's a question of priorities."

Weinberger had signed several directives putting SDI in a special category as the Pentagon's top priority program.

Carlucci's first major staff change, made last week before he took over the job, was to remove Weinberger's chief arms negotiator, hard-liner Frank J. Gaffney Jr.

"It wasn't a policy reason," Carlucci said. "As secretary of defense I need to be fully comfortable with the people who work for me.

"I told the senior staff this morning that there would be no purges in the Pentagon," he said.

He implied, however, that there could be additional changes among top staff. When asked about pending actions, Carlucci replied, "Any moves that have been made are in the presidential {appointee} areas. It would not be appropriate for me to comment on them until the president has made a decision."