North Vietnam's foreign minister said in a broadcast over Hanol's official news agency that the bombings posed "a grave threat to the work of the Paris conference on Vietnam."

He said U. S. warplanes "repeatedly attacked many populated areas, communication lines and economic establishments in Quang Bing and Ha Tinh provinces" in the southern-most corner of North Vietnam, which borders Laos below the 19th parallel. "Many flights"

of American planes "fired rockets on the periphery of Hanoi," he added.

Announcing the raids on Saturday, Defense Secretary Melvin R. Laird had said they were "limited duration protective reaction air strikes against missile and antiaircraft gun sites and related facilities in North Vietnam, south of the 19th parallel"-far to the south of Hanoi.

But an Agence France-Presse report from Hanoi Saturday said the city "shook today from the blast of bombs only 25 miles away."

The French news agency reported that "loudspeakers blared reports that United States planes were operating within 30 miles of the city. They warned the population to be on full alert. By mid-morning, Hanoi seemed back to normal, the streets busy with innumerable bicycles and small trucks, and children holding hands, walking to school as usual."

The operation was so politically sensitive that the U. S. Command imposed a news blackout and refused comment on two charges by Hanoi:

That five U. S. bombers and one helicopter were shot down during the around-the-clock bombing of North Vietnam that began before dawn Saturday, Saigon time.

That there were 34 "old persons and children" killed during the raids.

Highly placed U. S. sources said the Nixon administration was willing to gamble on the political consequences of resuming heavy raids over North Vietnam because some U. S. officials were unable to resist going after what they considered to be lucrative targets.

Laird did not say what the "related facilities" were, but sources identified them as supply dumps piled high with war materials awaiting shipment through Laos into Cambodia and South Vietnam.

The sources said the Pentagon seized on the downing of an unarmed American reconnaissance jet on Nov. 13 as justification for retaliatory raids, although a Pentagon spokesman said that since then other U. S. reconnaissance planes had been fired on but not hit. But the U. S. Command in Saigon had insisted since the Nov. 13 episode that it had no reports of American aircraft being fired on.

Laird added as justification the need to protect the lives of American pilots flying combat missions "throughout southern Laos."

Capitol Hill responded to the raids with criticism. Senate Majority Leader Mike Mansfield (D-Mont.) called them "a renewed involvement" that could delay settlement of the war.