Commerce Secretary C. William Verity yesterday declared that Japan's continued killing of whales undermines an international whale conservation pact, opening the possibility of U.S. trade sanctions on $550 million of Japanese fish products imported to this country.

The Reagan administration said it stepped in after the International Whaling Commission questioned the benefits of a Japanese research program that allows its fishermen to kill 300 whales this year for scientific purposes.

Japan stopped commercial whaling last spring under a 1984 agreement with the United States, but substituted the research program amid pressure from its powerful whaling industry.

Rep. Don Bonker (D-Wash.) hailed the Commerce Department statement and termed "patently false" Japan's claim that it is hunting whales for scientific research.

"If that is an example of Japanese science, the Sony Walkman would be the size of a suitcase," said Ken Cook of the World Wildlife Fund.

Verity's declaration immediately triggered a U.S. law that would reduce by half Japan's allocation of fish in U.S. waters. Because Japan has no allocation this year, Commerce Department officials acknowledged that the move yesterday was largely symbolic.

The declaration does, however, set the stage for a decision that the president will have to make within 60 days under another law that could limit or ban imports of Japanese fish products.

The United States imports about $550 million a year in fish products from Japan, including $110 million in pearls and pearl jewelry. Fish products are one of the few areas in which the United States maintains a positive balance of trade with Japan, selling $1.1 billion to the nation last year.

The whaling issue adds another point of friction to U.S.-Japan relations, already burdened by trade battles over a variety of products ranging from beef to semiconductors. Japanese fishing practices are under attack by groups of West Coast fishermen who accuse Japan of illegally fishing in U.S. waters.

The Americans made videotapes last month that showed seven Japanese fishing trawlers with their nets unfurled inside the U.S. 200-mile limit near the Aleutian Islands.

As a result, the State Department asked Japan to allow U.S. inspectors to board Japanese fishing vessels in international waters to determine if they had been fishing illegally in American waters.