MEXICO CITY—Mexico has significantly reduced imports of U.S. gasoline since President Andrés Manuel López Obrador took office Dec. 1, as a government crackdown on fuel theft has jammed up distribution and caused widespread gasoline shortages.

Seaborne gasoline imports from the U.S. Gulf Coast have averaged about 364,000 barrels a day under Mr. López Obrador, a 25% decline from December 2017 and January 2018, according to research firm ClipperData.

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