The United States had "numerous corroborative evidence and sightings" that Libya had fired missiles at U.S. planes before the United States launched a retaliatory attack, Defense Secretary Caspar W. Weinberger said yesterday.

Weinberger did not disclose how U.S. forces knew that Libya fired surface-to-air missiles, and all public accounts of the engagement have come from officials here, not eyewitnesses. There was confusion in administration reports as to whether four or six Libyan missiles were fired.

Defense Department officials said the evidence of missile launches came from several sources, including Navy and National Security Agency planes training their sophisticated surveillance devices on Libya's lone SA5 launch site.

Those devices are capable of intercepting voice commands to fire the missiles, detecting electronic impulses from the missiles as they are fired and tracking them by radar.

"This is a completely different thing from Gulf of Tonkin," one official said, referring to the still-disputed Vietnamese torpedo-boat attack on a U.S. Navy destroyer that prompted Congress to sanction a U.S. combat role in Southeast Asia.

"It's very easy to monitor when they launch missiles, especially when all your electronic warfare assets are focused on one area," the official added. "There's no doubt what went on."

Despite that note of certainty, several questions about yesterday's engagement remained unanswered last night.

It was unclear from U.S. accounts how many missiles had been fired, whether any pilots visually spotted the SA5s, how the pilots avoided them or how close the missiles came to striking U.S. planes.

The Soviet-made SA5, recently installed in Libya, is slower than most combat planes aboard U.S. aircraft carriers and useful mostly at high altitudes. It is considered a greater threat to relatively slow high-flying craft, such as the Airborne Warning and Control System (AWACS) radar plane, than to fast fighter jets.

U.S. officials reported no U.S. casualties but said they do not know how many Libyans were hurt or killed.

Weinberger and White House spokesman Larry Speakes said that there appeared to be no survivors on the first missile boat destroyed by U.S. Harpoon missiles. The second boat also appeared "pretty well destroyed," one official said.

Officials said they did not know if any Soviets were injured in the U.S. attack against the SA5 radar site. But they said they believed there were no casualties -- Libyan or Soviet -- there. U.S. intelligence had reported that Libyans were running the site with help from Soviet technicians.

* It was unclear how the two Libyan boats had threatened U.S. forces, if at all. Weinberger said one came within 38 miles of a U.S. vessel, the range of missiles carried aboard the fast patrol boats. He did not say whether the second ship attacked was within such range or whether either ship had "locked on" U.S. vessels with radar.

"It was heading toward the area where our ships are, after hostile acts had been committed by its country, and we don't subject our people to risks of that kind in this sort of situation," Weinberger said of the second boat. "And so it was attacked."

* Officials did not say at what level the retaliatory raid was cleared. While the three-carrier fleet assembled in the Mediterranean Sea had been authorized March 14 to fire if fired upon, the raid came several hours after the SA5 launchings and was presumably discussed again in Washington.

"It's been pretty well planned all along," one official said, meaning the U.S. response if fired upon. "But I'm sure the final plan was checked out at the top."

* Few officials would predict whether and how the conflict might escalate. U.S. officials said any Libyan planes or vessels approaching U.S. ships, which were to continue maneuvers, would be presumed to have hostile intent.

As to likely Libyan response, one administration official said, "I think they'll hit us in the United States."