On Feb. 22, Japan notified the International Whaling Commission that it planned to kill 50 minke whales and 50 sei whales on top of the 100 minkes it has been hunting annually in its research program. Japan also kills 440 minke whales in Antarctic waters each year.

The small, wedge-shaped minke whales are relatively plentiful. But sei whales, which grow to 60 feet and 25 tons and were heavily hunted into the 1960's, are listed as endangered by international conservation agencies and the United States.

An official representing a country on the whaling commission confirmed details of the Japanese plan, which was distributed to reporters by the World Wildlife Fund, a private environmental group.

In the past, the commission, established under a 1946 treaty regulating whale hunts, and many countries have criticized Japan's large- scale hunting of whales for research. But the harvest is not illegal or in violation of the whaling treaty.

Japan has strongly defended its research hunts, contending that most whale species are not endangered and that in some cases they are overabundant and depleting fish stocks. The Japanese say that the killed whales are dissected to provide a better understanding of their feeding habits and biology. The meat is sold in Japanese supermarkets and restaurants.

Yesterday, American officials said that an expanded whale hunt by Japan would bring condemnation from the United States.

"We strongly oppose Japan's lethal scientific whaling activities," said Rolland A. Schmitten, the American representative on the whaling commission and assistant administrator for fisheries of the National Oceanic and Atmospheric Administration. "This is de facto commercial whaling. If we confirm that they've added any additional whales or new species, we would intend to express our extreme disappointment to the Japanese government."

A year and a half ago, the Clinton administration threatened to impose economic sanctions against Japan the last time it expanded its research haul, adding Bryde's and sperm whales, species that were once heavily hunted and whose numbers today remain unclear.

Once Japan's plans are officially released, the Bush administration will consider whether to consider sanctions or other moves, Mr. Schmitten said.

The plans for an increased harvest come as Japan prepares to push the whaling commission to consider a new management plan for whales that would include the first resumption of commercial whaling since a moratorium in 1986.

"The Japanese government is attempting to prop up its whaling industry while it sets about dismantling the global moratorium," said Richard N. Mott, the vice president of the World Wildlife Fund.

A statement released by the Japanese Embassy in Washington rebutted such claims, saying that Japan's "research program is a good-faith contribution to the scientific review of the effect of the moratorium on whale stocks." The effort by Japan to sway the commission is expected to culminate in May, when it will host the next meeting of the whaling commission in Shimonoseki, a traditional whaling village.