Six U.S. senators urged President Donald Trump’s administration on Sunday to drop a plan to buy Russian diesel to help bring down fuel prices.

Trump said Friday that he had reached an agreement with Russian President Vladimir Putin by phone. He said Russia would immediately supply more than 300,000 tons of diesel to U.S. and global markets, with a further 500,000 tons in November, 1 million tons soon afterward and another 3 million tons later.

The four Republicans and two Democrats said the administration had neither consulted Congress nor provided the explanation they say the law requires. The signatories were Republicans Susan Collins, Lisa Murkowski, Thom Tillis and John Curtis, and Democrats Jeanne Shaheen and Richard Blumenthal.

Tillis also said there was a legitimate case for arguing that the president would not be following the law if the purchase went ahead. The announcement followed legislation passed weeks earlier directing Trump to impose substantial tariffs on major importers of Russian oil or natural gas, subject to an exception for some countries.