A bipartisan group of senators has sent a letter to the U.S. Trade Representative Michael Froman urging the Obama administration not to make tariff concessions to Japan during the Trans-Pacific Partnership trade talks.

The letter, dated Saturday and signed by 15 senators led by Michael Bennett, a Colorado Democrat, and Charles Grassley, an Iowa Republican, "asked for assurances that the TPP negotiations will not be concluded until Japan agrees to eliminate tariff and non-tariff trade barriers for agricultural products," the National Pork Producers Council said the same day.

Tokyo and Washington are jousting over Japanese duties on five "sacred" farm product categories — rice, beef and pork, wheat, dairy and sugar — that Tokyo wants to retain under the TPP, which is based on the principle of abolishing all tariffs.

"The senators pointed out in their letter that, if Japan is allowed to claim exceptions for sensitive products, other TPP countries inevitably will demand the right to do the same," the council said.

A number of U.S. agricultural organizations agree with the sentiments in the senators' letter, the council added.

Among them, Bob McCan, president of the National Cattlemen's Beef Association, was quoted as saying, "It is fundamentally important that all TPP members, including Japan, abide by the same terms of TPP as the other members."

Ministers from the 12 countries involved in the TPP talks kicked off four days of talks in Singapore on Saturday seeking to nail down a broad agreement after missing their end-of-2013 deadline.