The Soviet Union's most advanced booster rocket, being promoted for sale to the West and sometimes used to fly the Soviet version of the space shuttle, exploded seconds after liftoff last week and destroyed much of its launch pad.

The official Soviet news agency Tass yesterday reported the accident involving a Zenit booster, which it said "broke up" at the Baikonur Cosmodrome in the Central Asian republic of Kazakhstan on Oct. 4.

The three-sentence dispatch did not identify the rocket's payload, but Western analysts said it likely was a spy satellite.

The accident could affect the efforts of the Soviet space program, which has suffered technical and funding setbacks recently, to attract Western customers for its launch services, the analysts said. It also challenges Soviet assumptions about the safety of the Zenit's liquid fueled rocket engines in boosting their version of the manned shuttle.

No one was injured in the blast, Tass said, but a commission is investigating it, and officials said further Zenit launches have been put on hold.

Nikolai Semyonov, a spokesman for Glavkosmos, the Soviet space agency, told the Associated Press in Moscow that one of the two Zenit launch pads was "nearly totally destroyed" and its satellite payload ruined.

In the past, the Soviets have not publicized space accidents so close to the ground until decades after they occurred. Western analysts said the Soviets probably decided to go public so quickly for practical reasons: to prove their credibility in the commercial marketplace, and because of the increasing accessibility of the remote Baikonur complex to Western visitors and commercial satellite cameras.

"It's astonishing that they announced it. After a week of intense debate, it looks like the publicity people won over the security people," said James Oberg of Dickinson, Tex., an aerospace engineer and author of books on the Soviet space program.

He said he assumes the Zenit's payload was an electronic intelligence satellite like one launched by another Zenit last May and on some of the 19 other flights for which the Zenit has been used as a single booster. The Zenit also has flown strapped onto the Soviet superbooster Energiya, which carries the Soviet shuttle to orbit.

With "everybody trying to cut deals for cash . . . there has been a lot of panic in the Soviet military establishment that this would let the secrets out," Oberg said, adding that this military resistance was probably the reason for the Soviets' seven days of silence.

The U.S. Space Command, which monitors Soviet launch and orbital activities, "does not comment on other countries' launches, either their successes or their failures," said Maj. Thomas A. Niemann at the command's facility near Colorado Springs, Colo.

The Soviets' candor may be more significant than the explosion of the Zenit, Oberg said. He noted that the market leader, the French-built Ariane rockets, and American boosters have all suffered explosions.

President Bush cleared the way in August for a U.S. company to help build a spaceport in Australia to launch Zenit rockets carrying American-built satellites.

The Zenit carries a 12-ton payload and uses the Soviet Union's most advanced propulsion system, a liquid-fueled engine called the RD-170.

The U.S. National Aeronautics and Space Administration and the Soviets have long debated the safety of solid versus liquid-fueled boosters for manned spaceflight. NASA uses both on its space shuttles, but it was a solid-fuel booster that failed in the 1986 Challenger accident. The Soviets chose liquid-fueled engines on the assumption that they could be turned off or controlled for added safety.

"This explosion blows that assertion out of the water," Oberg said. An explosion on the launch pad "would have been the maximum disaster possible if it had been the shuttle. Their ejection-seat people will have to look at this carefully."

Scientist Nicholas Johnson of Teledyne Brown Engineering in Colorado Springs agreed that the accident could affect the Soviet shuttle, but he added: "The next flight is not planned until late next year. That gives them plenty of time to figure this out and fix it."