KUBINKA AIR FORCE BASE, U.S.S.R. (AP) _ U.S. Defense Secretary Frank Carlucci boarded the Kremlin's new secret bomber and watched war games on tours Tuesday of two bases that had been off- limits to Westerners.

The visits to Kubinka, including the bomber NATO calls the Blackjack, and to an army artillery division were unprecedented for an American official, Carlucci said later, but did not persuade him that the Soviets had made moved from an offensive to defensive military posture.

''We, for example, have seen no change in force structure nor have we seen any change in the resources going into the Soviet military establishment,'' he told a news conference back in Moscow. The Pentagon chief said he would continue seeking a 2 percent increase after inflation in the U.S. military budget.

During his visit to the air base 31 miles west of Moscow, Carlucci spent about 10 minutes in the cockpit of the long-range Blackjack bomber. Foreigners had seen it previously only in drawings and reconnaissance photographs.

Marshal Sergei F. Akhromeyev, the Soviet military chief of staff, inspected a B-1 bomber last month at Ellsworth Air Force Base outside Rapid City, S.D.

Soviet military officials were much more forthcoming to Carlucci than to foreign journalists about the sleek white plane with a sharply pointed nose.

They kept reporters and photographers behind a rope barrier about 50 yards from the plane and refused to tell them anything about it.

''Why do you want to know about the bomber? Why frighten people?'' asked Carlucci's host, Col. Gen. Boris F. Korolkov.

Korolkov, first deputy commander-in-chief of the Soviet air force, refused to identify the aircraft, whose only markings were a red star and a number 12 on the tail.

Soviet Military Power, the Pentagon reference book, says the four-engine bomber is the world's heaviest and largest but does not give dimensions.

Jane's All The World's Aircraft, a respected British publication, says the plane is known in the Soviet Union as the Tupolev strategic bomber, is 177 feet long, has a wingspan of 182 feet and weighs 275 tons.

The plane has been under development for 10 years, can fly at twice the speed of sound, has a range of 4,500 miles, and can carry bombs and cruise missiles, according to Jane's.

It and the U.S. Defense Department had predicted Blackjacks would be deployed this year. Korolkov said they became operational ''recently.''

Carlucci inspected a MiG-29 Fulcrum fighter first, then turned to the nearby bomber. As he did so, two Blackjacks emerged from patchy clouds and roared by.

The American visitor inspected the Blackjack's underbelly, then climbed orange stairs through the open bomb-bay doors and entered the cockpit.

He told reporters later he could not reveal technical details of the aircraft but compared it to the B-1 in the ability to fly at low altitudes and carry large payloads.

''I'm not a qualified intelligence officer,'' said Carlucci, a former senior CIA official. ''I'm not a qualified observer. They let me sit in the cockpit but I couldn't tell one instrument from another.''

He said his four-day visit to the Soviet Union, which began Monday, is largely symbolic, ''a way of continuing the dialogue between our two military establishments.''

At Kubinka, Carlucci also inspected an Ilyushin-78 tanker plane and Mi-26 helicopter, and watched a display of aerobatics by four MiG-29s.

Carlucci then traveled 12 miles southwest to the headquarters of the Taman Motorized Rifle Division to watch war games. Defense Minister Dmitri T. Yazov, Carlucci's host, stood next to him on a two-story building overlooking a huge field of mock battle.

Mi-24 helicopters flew past the building with guns firing. Artillery blasted targets in the field.

Infantry vehicles bounced over terrain pocked with shell craters and Yazov joked to Carlucci: ''It's not very comfortable to drive.''

Another guest was the defense minister's 10-year-old grandson, Oleg Losik. He rode around the Taman base on his 64-year-old grandfather's lap, next to Carlucci in the back seat of a black limousine.

When the games were over Carlucci asked Oleg if he wanted to shoot like the soldiers, but the boy appeared too shy to answer.

Oleg told a reporter later, wearing an oversized military officer's jacket to keep warm, that the war games were ''very interesting'' and only the second he'd seen.

On Carlucci's schedule for Wednesday is a visit to Black Sea fleet headquarters on the Crimean Peninsula.