The nation's antimonopoly watchdog has issued a warning to two gasoline retailers, including the Japanese unit of U.S. retailer Costco Wholesale Corp., over a price war that it said could have threatened the activities of competitors.

Costco Wholesale Japan Ltd. and Japanese oil products retailer Baron Park Co. sold regular gasoline at their gas stations in Tokoname, Aichi Prefecture, at between ¥86 and ¥87 per liter for 10 days through Nov. 27. This compares with retail gasoline prices that averaged ¥123 to ¥129 in the prefecture in the weeks through early December, according to the Natural Resources and Energy Agency.

The prices were "excessively below" the costs of procuring the gasoline, the Fair Trade Commission said in a statement Thursday.