Amazon.com Inc. is barring foreign sales of seeds into the U.S.–-and e-commerce company Wish will ban the sale of seeds—after thousands of suspicious packets, many postmarked from China, arrived at households around the world this summer.

The move by Amazon comes as the mystery seeds led U.S. officials to raise alarms about the ease with which seed sales can occur on e-commerce sites, creating potential threats to U.S. agriculture.

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