Japan, Mexico and Canada may participate in a Pacific-region trade accord, although they won't be able to alter any aspects of the deal already established, a senior U.S. negotiator said.

There is still an opportunity for the three countries to join, said the official, who spoke to reporters on the sidelines of trade discussions near Dallas.

Representatives from nine countries are meeting this week for the latest round of negotiations in the Trans-Pacific Partnership free-trade accord. The countries working toward a final agreement with the U.S. are Australia, Brunei, Chile, Malaysia, New Zealand, Peru, Singapore and Vietnam. Officials from Japan, Mexico and Canada have expressed interest in joining as well. Adding the three nations would create the largest such trade agreement in U.S. history.

Existing participants in the talks are conducting their own discussions with the candidate countries, and no final decision has been made on expanding the group's membership, the official said.

Negotiators working on an accord to boost trade for small and midsize businesses have concluded their talks and won't have to participate in further multilateral meetings, the U.S. official said.