Secretary of State George P. Shultz yesterday defended the administration's military and paramilitary moves in Central America before a skeptical Senate Foreign Relations Committee, saying that "something has now begun to happen" diplomatically in the region after the U.S. show of muscle.

Shultz broadly implied that the U.S. display of strength had provided "incentives" for Cuba, Nicaragua and leftist guerrillas in El Salvador to change their course. He cited recently expressed interest in dialogue by all three groups, saying this followed the sending of "messages" that "a victory by the far left and its foreign supporters through armed force is not in the cards."

Shultz's testimony followed two weeks of intense political controversy about the U.S. course in Central America. This was reflected in unusually outspoken skepticism and concern from committee members of both parties.

Shultz portrayed the large-scale, U.S.-Honduran ground, air and naval exercises near Nicaragua as designed in part to "underline the American capability for deterrence" in the region and "in that sense, designed to make a point."

Vice Adm. Thomas J. Bigley, director of planning and policy for the Joint Chiefs of Staff, who accompanied Shultz, testified that the timing of the military exercises had been moved ahead because "the situation was developing" in the area, including Honduran concern about Nicaraguan military forces poised near its border.

The United States is prepared to aid Honduras in case of an invasion from Nicaragua, Shultz said in response to a question from Sen. Christopher J. Dodd (D-Conn.). He added that "in what way depends on the circumstances."

The Hondurans "realize that we support them but they don't have a commitment from us to let them pull our trigger for us," Shultz said.

Sen. Claiborne Pell (R.I.), ranking committee Democrat, told Shultz that "we are on a slippery slope in Central America, and the American people want to know where we are headed." Pell added, "You can't scare the Cubans and Nicaraguans and still expect to reassure the American people that the United States will not be drawn into another bloody conflict."

Pell sought in vain to obtain a definitive response from Shultz to a Washington Post report from Honduras last Thursday. The report stated that "a major aim of the rapidly escalating U.S. military presence in and around Honduras is to provide a protective shield for this country as U.S.-backed rebel forces operating along the border with neighboring Nicaragua expand their efforts to overthrow the Sandinista government there."

Shultz said it is not the U.S. government's aim to overthrow the Managua regime, though he conceded that this "certainly is" the aim of counterrevolutionary Nicaraguan forces, known as "contras," which the United States is reported to be backing.

As far as U.S. military exercises operating as a shield for the contras, Shultz said only that the U.S.-Honduran maneuvers are basically for training and that "there is nothing in the plan that represents an operational program . . . . There is no intention to engage anybody tactically."

"If a hostile situation develops, our forces will withdraw," Shultz said. He went on to say that "they'll defend themselves but they'll withdraw."

Bigley said the conduct that Shultz outlined for the U.S. maneuver force conformed to "standard peacetime rules of behavior."

The admiral, as well as Defense Department spokesman Henry Catto in a Pentagon news briefing, portrayed the hailing of a Soviet cargo ship by a U.S. guided-missile destroyer off Central America last weekend as commonplace.

"It was not an unusual occurrence," Bigley told the committee. Catto told reporters, "It is not an uncommon practice for inquiries to be made of ships that are sailing in the vicinity of a battle group."

Nonetheless, the Pentagon spokesman said it would not be routine for all Soviet-bloc vessels to be queried when they are near Central America in the future. "That's going to be strictly up to the commanders on the scene," Catto said.

In a related development, more than 50 liberal Democratic congressmen asked the chairmen of two House committees to deny the Pentagon permission to divert money from other programs to pay for the military exercises, which they said "can only deepen tensions throughout" Central America.

At the outset of the Senate hearing, Shultz heard an expression of "grave concern" from Chairman Charles H. Percy (R-Ill.) about the administration's failure to consult Congress adequately and about an impression that its policies in Central America are not well thought out or coherently presented.

Some Republicans, including Sen. Richard G. Lugar (Ind.), who charged that "an unparalleled military juggernaut" in Nicaragua threatens Central America and even Mexico, and requires a strong U.S. response, defended administration policy.

While never formally acknowledging that the United States has been supporting the anti-government rebels in Nicaragua, Shultz implicitly approved of their activities as "hardly surprising" in view of the policies of the Managua regime. He listed "a popular insurgency inside Nicaragua" as one of the positive factors bringing about a degree of diplomatic change among leftist forces.

In the diplomatic arena, Shultz said "the rhetoric has changed in the last month or so" on the part of Cuban and Nicaraguan authorities. He did not say whether he anticipates a more basic and tangible shift in their activities.

Another senior State Department official familiar with the recent mission to Managua of special U.S. envoy Richard B. Stone said Stone found "no indication that the Nicaraguans are ready to go beyond generalities" at this point.