WASHINGTON, May 23 (Reuters) - The U.S. government willrequire meat packers to explicitly list the origin of beef, porkand chicken sold in U.S. grocery stores, it said on Thursday, aregulation intended to resolve years of disputes with Canada andMexico.

But Canada's agriculture minister said it will continue tofight the country of origin labeling rules (COOL) which he saidhad contributed to a sharp decline in Canadian cattle and pigexports, setting up a possible trade war.

 The Obama administration unveiled the new rule on the finalday to comply with a World Trade Organization decision, issuedin June 2012 that upheld complaints by Canada and Mexico.

Under the new regulation, labels will carry labels such as"Born, Raised and Slaughtered in the United States" for U.S.animals. Meat from other countries could carry labels such as,"Born in Mexico, Raised and Slaughtered in the United States."

No commingling of meat from various nations will be allowed.

"The United States remains committed to ensuring thatconsumers are provided with information about the origin ofmuscle cut meats they buy at the retail level," the U.S.

Department of Agriculture said in a statement.

The revised rules take effect immediately; however the USDAwill allow a six-month grace period for compliance.

In the original rule, which took effect in March 2009,packages could carry labels saying the meat was from the UnitedStates and other nations.

Canadian Agriculture Minister Gerry Ritz said the changesare disappointing, and don't comply with WTO rules.

Ritz said one of Canada's options under consideration isasking the WTO to approve retaliation against U.S. products,but he would not say which products Canada would most likelytarget. In the past, he has said Canada would likely aim at moregoods than just U.S. meat.

"We have no intention of backing off or backing down, if theAmericans think this is a game of chicken," Ritz said. "We willdo everything within our power to make sure they understand thatboth Canadian industry as well as American industry (is)totally rejecting what they came forward with today."

COOL was backed by U.S. consumer groups and some U.S. farmgroups. It was opposed by trade groups representing U.S. cattleand hog producers and food makers.

"People have the right to know where the food they feedtheir families comes from," said Wenonah Hauter, executivedirector of Food and Water Watch.

But Scott George, a Wyoming cattle producer and president ofthe National Cattlemen's Beef Association, said the USDA'saction risks retaliatory action against U.S. beef, which wouldbe "devastating" for farmers.