Japanese and U.S. trade negotiators have failed to make major progress in bridging gaps on auto trade as talks continue on sealing a Pacific Rim free trade initiative, a Japanese official said.

"As we entered a crucial stage, we had many difficult moments," Takeo Mori, Japanese ambassador in charge of economic diplomacy, told reporters in Washington on Friday after talks with Wendy Cutler, the deputy acting U.S. trade representative.

Mori and Cutler began their latest round of talks on auto trade Wednesday as a separate channel from bilateral negotiations on farm products related to the 12-nation Trans-Pacific Partnership deal. The U.S. has historically had a difficult time getting access to Japan's auto market, a source of trade friction highlighted by the stark difference in market share.

Mori, deputy director general of the Economic Affairs Bureau of the Foreign Ministry, said he and Cutler thrashed out the legal language of an envisioned agreement. The Office of the U.S. Trade Representative said in statement Friday that Cutler and Mori "continued to make steady progress and narrow the remaining differences."

In the TPP-related auto talks the U.S. has urged Japan to open its market further, saying nontariff barriers, such as regulatory standards, prevent American carmakers from expanding sales in Japan. Tokyo usually attributes U.S. sales woes to a lack of models that appeal to its finicky consumers.