Radio Hanoi reported that "wave after wave" of U. S. bombers attacked North Vietnam early Saturday, including a prisoner of war camp where the station said there were "a number of U. S. prisoner of war casualties." Considerable numbers of North Vietnamese civilians had been killed as well, the station said.

BULLETIN 3:30 A. M.

Secretary of Defense Melvin R. Laird announced early today that U. S. warplanes are hitting North Vietnamese missile and antiaircraft gun positions "in response to attacks on our unarmed reconnaissance aircraft."

Radio Hanoi said at least three U. S. jet bombers and one helicopter were shot down by North Vietnamese defenders during the attack. It said later reports from other areas might increase the plane toll.

Interrupting its regular broadcasting schedule for the bulletin report, the official North Vietnamese radio station said the air attack started at 2:30 a. m. Hanoi time and lasted more than an hour, with the American planes bombing targets in an area ranging from the port city of Haiphong to Hoa Binh province southwest of Hanoi. The Vietnamese language broadcast, monitored in Hong Kong, did not give the name and the location of the prisoner of war camp where it said there were American bombing casualties.

In Saigon, an official spokesman for the U. S. command said he had no reports of American bombing raids against North Vietnam. He said of Radio Hanoi's report, "If it in fact is true, I have not seen or heard it. Officially, I have nothing on it."

The Radio Hanoi charge followed an announcement in Saigon that the U. S. Seventh Fleet had sent a second carrier steaming into the Tonkin Gulf and had doubled its strike capability off the coast of North Vietnam.

The intent of that action was not immediately clear, but sources said they knew of no raid over North Vietnam. They said most of the Navy planes were striking against North Vietnamese supply routes running through Laos.

The 12,000-ton carrier Hancock, which came from its home base in Alameda, Calif. joined the carrier Oriskany in the Tonkin Gulf to launch a strike over a 2000-mile stretch of the Ho Chi Minh trail.

Most of the 150 war planes from the two carriers joined up to 400 air force aircraft flying from bases in Thailand and South Vietnam in what the U. S. command called a campaign to stem the flow of North Vietnamese war material southward.

U. S. spokesmen said other supersonic jest from the Hancock equipped with spy gear including highly sophisticated cameras streaked into North Vietnam to photograph supply buildups awaiting shipment southward.