TOKYO, Japan (CNN) -- Anti-whaling protesters hurled containers of butyric acid at a Japanese whaling ship in Antarctic waters, injuring four crew members, a Japanese official said Monday.
Activists from the Sea Shepherd Conservation Society hurl objects on to the Japanese ship Nisshin Maru.
The Sea Shepherd Conservation Society acknowledged the Sunday incident, saying it had lobbed more than two dozen bottles of rotten butter at the Nisshin Maru, "sending a stench throughout the whale killing ship that will remain for days."
Butyric acid is found in rotten butter.
The Sea Shepherd boat had to move a half-mile away from the whaling ship because "it stinks too bad to remain any closer," activist Todd Emko of New York said in a statement from the group. 
Watch protesters toss objects at the ship »
The conservation group said it also threw packets of a slippery chemical on to the deck of the ship, making it difficult to cut up whales.
The unnamed substance becomes more slippery when mixed with water so it will be difficult to wash off the deck, a Sea Shepherd statement said.
"I guess we can call this nonviolent chemical warfare," said Capt. Paul Watson of the Sea Shepherd boat, the Steve Irwin. "We only use organic, nontoxic materials designed to harass and obstruct ... whaling operations."
Itsunori Onodera, Japan's senior vice minister for foreign affairs, described the incident at an international seminar for African and Asian delegates to the International Whaling Commission.
He said the Sea Shepherd group threw bags of acid, rather than bottles of rancid butter.
Onodera told the audience that the crew complained of injuries afterward, but he didn't elaborate.
Sea Shepherd's U.S. office said it did not know of any injuries.
Sea Shepherd interferes with Japanese hunts in an attempt to reduce the number of whales taken. In January, two activists from the group were seized when they boarded another Japanese whaling ship.
Japan is lobbying a dozen members of the International Whaling Commission in Tokyo to support its much-criticized Antarctic whaling program. The effort comes before all 78 members of the panel this week in London, England. The commission is meeting to discuss reaching an agreement on whale conservation rules.
Japan's Ministry of Foreign Affairs and its Fisheries Agency are making their case to officials from Angola, Eritrea, the Republic of the Congo, Guinea, Ghana, Malawi, Tanzania, Palau, Micronesia, Cambodia, Laos and Vanuatu.
Japan has said it is conducting the hunt as research, calling the practice environmentally and scientifically sound. Watch why Tokyo is furious over the stink between whalers, protesters »
The country's annual hunt kills up to 1,000 whales a year; the Fisheries Agency insists it wants "sustainable whaling."
However, many in the international community -- particularly Australia -- say such hunts amount to needless slaughter. Critics have said calling these hunts research is a pretext for retrieving whale meat to be sold in markets and restaurants. E-mail to a friend 
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