New York (CNN Business) Amazon wants to turn its famous two-day delivery into just one-day on most Prime orders. But can warehouse workers handle it?

Amazon AMZN The company's plan to spend $800 million this spring to speed up deliveries for Prime members has sparked a tense back-and-forth between anexecutive and the leader of a major workers' union.

Stuart Appelbaum, president of the Retail, Wholesale and Department Store Union, said speeding up deliveries could be dangerous for Amazon fulfillment center employees. Appelbaum said workers are already struggling to keep pace and handle 200 to 300 orders per hour during a single 12-hour shift.

"Increasing fulfillment speeds means they need to hire more workers, under more sustainable speeds that don't put worker's lives in jeopardy," Appelbaum said Friday.

RWDSU, which represents workers in brick-and-mortar grocery stores, has advocated for Amazon workers in the past and is trying to help them unionize in some parts of the country. Amazon warehouse workers in the United States aren't unionized. RWDSU also criticized Amazon's plan for a second headquarters in New York City. That plan was eventually scrapped.

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