The U.S. Trade Representative's office said it was challenging Canada's revised tariff-rate quota allocations that were put in place last year after a previous dispute panel decision under the U.S.-Mexico-Canada Agreement on trade.

USTR said in a statement that Canada was unfairly using a market-share approach for determining TRQ allocations, and had imposed new conditions that effectively prohibit retailers, food service operators, and other types of importers from utilizing quota allocations.

"Although the United States won a previous USMCA dispute on Canada's dairy TRQ allocation policies, the Canadian government's revised measures have not fixed the problem," Ambassador Katherine Tai said in a statement.

Canada's trade minister, Mary Ng said in a statement issued in Ottawa that she was 'disappointed' in the U.S. move and would defend Canada's longstanding Supply Management system that protects Canadian dairy farmers from imports.

"We will stand firm against attempts to re-negotiate during this dispute settlement panel process," Ng said.

(Reporting by David Lawder; Editing by Andrea Ricci)