Anti-whaling activists hurled containers of rotten butter at a Japanese whaling ship in Antarctic waters Monday, lightly injuring several crewmembers, Japan's government said.

Protesters aboard a boat operated by Sea Shepherd threw the objects — containing butyric acid, produced by rotting butter — at the Nisshin Maru whaling ship, which is conducting Japan's research whaling program.

"We strongly condemn this," Chief Cabinet Secretary Nobutaka Machimura said. Japanese officials said at least three crewmembers were injured lightly. It was not clear if they were hit with the bottles.

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Japan planned to issue a formal protest to the Netherlands, which licenses the activist boat, the Steve Irwin, said Itsunori Onodera, senior vice minister for foreign affairs.

"This kind of illegal act should not be tolerated," Onodera said at a small whaling conference that Japan was hosting Monday with African and Asian nations in Tokyo.

Sea Shepherd and other anti-whaling protesters have continually harassed the Japanese whaling fleet in an effort to interfere with Tokyo's hunting season, during which it takes hundreds of minke whales in the Antarctic.

Japan has accused the activists of terrorist tactics, but Sea Shepherd called on Japan to stop its hunt, in a statement issued after the confrontation.

The group said it threw more than two dozen bottles of rotten butter — which has a rancid odor — onto the Japanese ship, as well as packets with an unspecified slippery chemical onto the deck to interfere with whale flensing.

"I guess we can call this nonviolent chemical warfare," said Sea Shepherd leader Paul Watson in the statement.

The statement made no mention of injuries on the Japanese ship.