A group of celebrities headed by hip-hop producer Russell Simmons called on the Obama administration to resist signing a trade agreement with Japan until an annual dolphin slaughter is banned in a fishing town there, according to The Associated Press

The letter asks U.S. Ambassador to Japan Caroline Kennedy to urge President Obama to hold off on signing the Trans-Pacific Partnership until the practice is stopped. 

"The world is looking to you, Ambassador Kennedy, and to our government to send a clear message to Japan that this atrocity must be banned now," the letter reads. 

Kennedy spoke out on Twitter ahead of the annual hunt last month, saying the U.S. government opposes “drive hunt fisheries” and branded the practice as inhumane. 

The cause gained an increased amount of attention after the 2009 documentary “The Cove” highlighted the practice.

The letters says at least 600 dolphins have been killed since last September. Simmons spoke out about the issue last month as well. 

The letter goes on to say the celebrities do not oppose the trade agreement but believe it should be used as leverage in negotiations to end the killing. 

The letter comes as Obama has sought fast-track trade promotion authority from Congress in order to expedite passage of the trade bill through Congress once it is negotiated — something Senate Majority Leader Harry Reid (D-Nev.) and others have resisted. 

The trade agreement includes the United States and Japan as well as 10 other Asian and South American countries and would affect about 40 percent of the world’s gross domestic product, according to the administration. 

Celebrities signed on the letter include Sean Penn, Cher, Susan Sarandon, Jennifer Hudson, Gwyneth Paltrow, Charlize Theron, Ellen DeGeneres and William Shatner, among others.