A Soviet nuclear attack on North America could kill "hundreds of millions" of people in what would be "the greatest catastrophe in history by many orders of magnitude," Gen. David C. Jones, chairman of the Joint Chiefs of Staff, said yesterday.

Jones painted that grim picture under what he called "the worst conditions" when asked by the Senate Foreign Relations Committee yesterday to estimate the casualties from "an all-out nuclear exchange" between the United States and the Soviet Union.

Sen. Claiborne Pell of Rhode Island, senior Democrat on the committee, asked that and related questions of Jones and Defense Secretary Caspar W. Weinberger as the senator probed the implications of President Reagan's rearmament drive.

Weinberger declined to estimate the casualties from a nuclear war, telling the committee that although there are organizations that seem to do little else but make such estimates, he did not know of one he could give as authoritative.

The defense secretary similarly stiff-armed questions about whether he believed the United States could win a nuclear war, confining his answers on the Reagan administration's philosophy to the intention of deterring war by strengthening the American military.

But Weinberger told the committee that the Soviets "by their actions must think it's winnable. . . . "

"When you look at the arsenal they have amassed over more than 21 years," Weinberger said, "I have to conclude that that is the assumption which underlies all of their thinking."

Pell, in noting the huge nuclear arsenals that both the United States and the Soviet Union are building, recalled to Weinberger former Soviet premier Nikita Khrushchev's rhetorical question: "Why make the rubble bounce?"

Weinberger said the Reagan administration is not striving for military "superiority" over the Soviet Union but just wants to "regain enough strength so we can deter an attack upon us. That is all we seek."

Asked by Sen. Joseph R. Biden Jr. (D-Del.) whether it was realistic to assume that a nuclear war between the United States and the Soviet Union would last a long time, Weinberger replied that "the Soviets have a refiring capability and a virtually unlimited supply of missiles."

Refiring in that context means launching one missile after another from the same silo. A "cold launch" keeps the launching equipment from being burned so it can be used again within a short time. The arms control agreement SALT II would limit launchers deployed on both sides, not the missiles for them.

"I don't know if it's realistic" to assume nuclear war would last more than several days, Weinberger told Biden, "but if this is the situation we are faced with, you cannot hope to deal with it or to deter attack unless you have the kind of capability that will enable you to respond." He said it appears the Soviets "are at least planning more than one strike."

Weinberger said the president's six-year program for strengthening the nation's nuclear forces would provide the needed deterrence. Reagan plans to build 100 B1 bombers; deploy 100 MX missiles, inserting up to half of them in existing Titan or Minuteman silos; put nuclear cruise missiles in the noses of attack submarines, and upgrade the warning and command network.

The administration's negotiating position with the Soviets on arms reduction would be "weakened very severely" if Congress rejected the president's B1 or MX proposal, Weinberger said.

Weinberger told the committee that last Sunday's Washington Post story suggesting that the United States and Saudi Arabia were putting together a massive military command network without the Senate knowing about it "came out of whole cloth. Nothing was kept from the Senate. There is no secret treaty." He added that it would be logical, however, for U.S. forces to use facilities in Saudi Arabia if they were sent there to protect the oil fields.