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By Collin Eaton

HOUSTON, May 31 (Reuters) - U.S. President Donald Trump's threats to tax Mexican imports could disrupt a long-standing cross-border energy trade, hitting U.S. consumers and refiners that use Mexican oil by boosting prices, and raising concerns about potential retaliation by the world's biggest buyer of U.S. energy products.

Mexico sends 600,000 to 700,000 barrels of oil to the United States every day, mostly to refiners that process that crude into gasoline, diesel and other products.

Mexico buys more than 1 million barrels per day (bpd) of U.S. crude and fuel, more than any other country, and analysts are concerned that retaliatory tariffs from Mexico could disrupt that trade.

"I can't see how the outcomes are going to be constructive," said Carlos Pascual, a former U.S. ambassador to Mexico who now helps run consultancy IHS Markit's global energy business

Trump on Thursday vowed to impose a tariff on all goods coming from Mexico, starting at 5% and increasing monthly until the surge of undocumented immigrants from across the border subsides.

Mexico and the United States, along with Canada, are trying to finish a broad free-trade agreement to replace the 25-year-old NAFTA deal. If implemented, the tariffs would begin June 10. So far Mexico has not said it would retaliate.

The imposition of tariffs may spur "retaliatory actions that impair the development of new markets," said a spokesman for Chevron Corp, adding the company supports free and fair trade. Chevron has opened 100 retail gasoline stores in Mexico since 2017.

Trade group American Fuel and Petrochemical Manufacturers warned tariffs could raise domestic fuel prices and jeopardize the proposed trade deal. The American Petroleum Institute said the tax could hurt the U.S. economy.

Tariffs could add $2 million to the cost of daily Mexican crude purchases by U.S. refiners, analysts at PVM Oil Associates said.

A sharp decline in supplies from Mexico could raise the cost of fuels overall if U.S. refiners are forced to buy heavier crude grades from further away, adding to shipping costs.

"The Trump administration is putting itself into a situation where it could create a much tighter heavy crude market that could impact the price of gasoline and force refiners to adjust their product slates due to shortages," said IHS's Pascual.

However, crude traders noted that most Gulf Coast refiners that buy Mexican crude are located in so-called Foreign Trade Zones, which allow them to avoid tariffs so long as the refined products are exported - though these refiners also supply U.S. markets.