Four Japanese whaling vessels owned by the government's Fisheries Agency left port on Saturday and headed for the northwestern Pacific to conduct whaling for research purposes.

The four ships, part of a six-vessel fleet, left from Innoshima in Hiroshima Prefecture, Shimonoseki in Yamaguchi Prefecture and Shiogama in Miyagi Prefecture.

The lead vessel left July 11 and the remaining ship is scheduled to depart on Monday.

The agency said the fleet plans to catch up to 160 whales during a two-month period through late September.

In addition to minke whales, the haul will include sperm whales and bryde's whales, which have not been hunted for 13 years, the agency said.

The agency said Japan is resuming the hunting of the two species because scientific data suggests their populations have recovered sufficiently for whaling to restart.

It said the hunting of the whales will allow scientists to gather information on the habitats of whales, the migratory routes they use, the quantity of marine creatures they eat and how pollution of their environment affects whales' habitats.

Meat from the whales caught will be sold commercially and is expected to be on the market by the end of the year, the agency said.

According to news reports, the United States on Friday urged Japan to halt its plan to expand whaling for research purposes.

Reuters quoted U.S. National Security Council spokesman P.J. Crowley as saying that U.S. President Bill Clinton and British Prime Minister Tony Blair strongly opposed the planned whale hunt in a joint letter sent earlier this week to Prime Minister Yoshiro Mori.

"We are strongly opposed to this program and we have urged Japan to reconsider its decision," Crowley was quoted as saying.

Earlier this month, the International Whaling Commission adopted a nonbinding resolution calling on Japan to abandon the research whaling program.

Japan maintains that research whaling is permitted under an international treaty and the meat is sold partly to finance the program.

Antiwhaling nations say Japan is using the IWC research whaling program as a loophole for commercial whaling.

The Fisheries Agency plans to send another research whaling fleet to the Antarctic Sea this winter.

Japan halted commercial whaling in compliance with an international moratorium in 1986 but has engaged in research whaling since 1987.