(Reuters) - A group of 38 U.S. senators asked the U.S. Environmental Protection Agency on Thursday to abandon plans to lower biofuel requirements for oil refiners in 2018, according to a letter made public by the office of Democratic Senator Amy Klobuchar of Minnesota.

The senators, including Republican Senator Chuck Grassley of Iowa, said in the letter that the EPA should restructure its proposal for the 2018 version of a rule that each year requires producers of oil-based fuels to include renewable fuels such as ethanol in their products. They said the agency should increase the amount of biofuels that must be included in the products.

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"The rule unjustifiably flatlines biomass-based diesel, reduces advanced biofuels and reduces the cellulosic biofuel blending target by about 25 percent," the letter said. "The final rule should address these shortfalls."

EPA spokesman Jahan Wilcox said in an email to Reuters: "We will respond to Senator Grassley and his colleagues through the proper channel."

(Reporting by Emily Flitter; Editing by Jonathan Oatis and Steve Orlofsky)