A bipartisan group of six U.S. senators on Sunday called on President Donald Trump's administration to "immediately reverse course" on a plan to purchase diesel from Russia as part of an effort to lower high fuel prices.

Trump stunned lawmakers on Friday when he announced on social media that he had struck a deal with Russian President Vladimir Putin during a phone call. Under the terms he described, Russia would immediately supply more than 300,000 tons of diesel to the American and global market, followed by an additional 500,000 tons in November and 1 million tons "immediately thereafter." Russia would deliver a further 3 million tons "within a short period of time" after that, he wrote. Taken together, the schedule he outlined would put roughly 4.8 million tons of Russian diesel into the American and global market.

Four Republican senators and two Democratic senators signed onto a statement released Sunday protesting the deal. They said the administration failed to consult with Congress or submit any explanation to justify its decision, as required by statute.

"The United States should not provide Putin with a financial lifeline while Russia continues to kill Ukrainian civilians. The Administration must immediately reverse course," the senators wrote.

The Republican signatories are Sens. Susan Collins of Maine, Lisa Murkowski of Alaska, Thom Tillis of North Carolina and John Curtis of Utah. The Democratic signatories are Sens. Jeanne Shaheen of New Hampshire and Richard Blumenthal of Connecticut.

Lawmakers Cite 2022 Ban and New Sanctions Law

Tillis, in an interview that aired Sunday on NBC's "Meet the Press," said there is a legitimate case for arguing that the president is not following the law if the administration goes through with its diesel purchase plan.

"We were very clear that Russia is the aggressor. Russia is the who can end this war by backing out of territory that they occupied," Tillis said.

Trump's announcement came just a few weeks after Congress passed legislation directing him to impose substantial tariffs on the top importers of Russian oil or natural gas, with an exception for countries that import less than 15% of Russia's natural gas exports and have taken significant steps to reduce those imports. The congressional effort was aimed at cutting the flow of money Russia receives from oil and gas sales to finance its war against Ukraine.

The lawmakers also noted that Congress explicitly banned Russian energy imports in 2022 to prevent the United States from financing the war. The dispute thus pits the administration's fuel-price rationale against statutes Congress passed with the stated aim of cutting off the energy revenue that finances Russia's war.

"Congress was clear: the U.S. should not import oil from Russia until it ends its aggression against Ukraine and ceases threats against our NATO Allies," the six lawmakers wrote in their statement. "President Trump's decision to allow Russian diesel imports to the U.S. runs directly counter to that law."

Administration Points to Energy Costs

Sen. John Barrasso, R-Wyo., said Trump is focused on reducing the cost of energy when asked whether the president's action would violate U.S. law.

"The president is balancing and has to make tough decisions," Barrasso said on CBS's "Face the Nation." He also encouraged Trump to enforce the sanctions law Congress passed last month.

"This is just another point of leverage for the president to use against Putin in an effort to get a long-term solution in that part of the world," Barrasso said.

Trump has recently sought to blame record U.S. diesel prices on Ukraine rather than the U.S.-led war in Iran, which has elevated world energy prices.

For now, the standoff turns on two open questions: whether the administration reverses course as the six senators demand, and whether Trump enforces the sanctions law Congress passed last month.

This story was originally featured on Fortune.com.