Courts worldwide have shown "astonishing leniency" in sentencing criminals for the theft or smuggling of material used to make nuclear weapons, a leading expert on nuclear proliferation said yesterday.

In his second annual report on efforts to halt the spread of nuclear weapons, Leonard S. Spector said, for example, that seven cases prosecuted successfully in the United States and three European countries last year involved Pakistan, which is "at the threshold of becoming a nuclear-weapons state," but that the sentences levied totaled 15 months and the fines totaled $16,000. The reason is probably the desire to remain friendly with a nation opposed to the Soviet Union in a strategically important area, Spector said.

He presented details of his report, "The New Nuclear Nations," at a news conference sponsored by Foreign Policy magazine and the Carnegie Endowment for International Peace.

Spector said evidence disclosed in the various trials indicated that contraband trade in "the nuclear netherworld" involves mechanical devices for bombs rather than radioactive materials and that governments rather than independent or terrorist groups are the main actors in "a constant attack on the system" of international safeguards.

"We're not up against a nuclear black market as such . . . . Weapons and weapons material does not seem to be available," he said. "These are national governments working to establish networks."

South Asia shows "the greatest increase in the danger of proliferation over the past year," Spector said, because of "very significant" advances made by Pakistan and continuing weapons capacity development in India. New large research and power reactors on line last year brought India's bomb-making capacity to 1,000 percent greater than it had in 1974, when it detonated a nuclear device, he said.

India maintains that its one detonation was for pealy five nations are openly members of the "nuclear club": the United States, the Soviet Union, Britain, France and China. Spector said he thinks that Israel, "the classic incident of silent proliferation," has 25 nuclear weapons, while South Africa probably has nuclear material for 10 to 15 weapons.

Spector is former chief counsel to the Senate Energy subcommittee on nuclear proliferation and is a senior research associate at the Carnegie organization.