Donald Trump says Russia will provide "the American and global marketplace" with nearly five million US tons of diesel after a phone call with Vladimir Putin.
The US president said the first tranche, comprising 300,000 tons, would be released immediately in a post on his own social network, Truth Social.
He said it would be followed by another 500,000 tons in November, with a further 1,000,000 tons "immediately thereafter".
Russia would then provide 3,000,000 tons "within a short period of time", he added. A US ton equates to 907kg, while a metric tonne weighs 1,000kg.
Mr Trump went on to say that "lower prices for Americans, especially our great farmers, ranchers, and truckers, is my greatest priority".
Ukraine's president Volodymyr Zelenskyy criticised the agreement, calling it a "gift to Putin".
He said the Russian leader is "relying on attempts to strike backroom deals to secure some form of pressure relief for Russia".
Mr Zelenskyy added: "Any easing of sanctions against Russia without a clear and lasting de-escalation agreement with Russia is an obvious weakness.
"It plays into Russia's hands - allowing it to kill more, wage war for longer, have even less respect for America, and inflict even greater losses and damage on the world."
The Kremlin said the phone call - initially expected to focus on the suspected plague death in Siberia - had also covered the war in Iran and prospects for resolving the "Ukrainian crisis".
Fuel prices have risen dramatically since the start of the US-Iran war, with Americans due to head to the polls for the midterm elections on 3 November.
The Treasury Department said it was immediately issuing a temporary license allowing Russian diesel to be supplied to the global market - but it is unclear when the fuel will become available.
Russian presidential envoy Kirill Dmitriev said that "Russia-US cooperation on diesel and energy will benefit the world".
It comes after Mr Trump signed a Russia sanctions law last month targeted at Russia's energy economy and other areas which fuel its war against Ukraine.
It applies to Russian officials, banks and a shadow fleet of tankers.
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The measures also ban all new US investment in Russia and the handling of sovereign debt.
The US has not imported Russian oil or gas since 2022, when then-president Joe Biden banned them following Moscow's invasion of Ukraine.
The national average for a gallon of diesel in the US was $6.23 (£4.71) on Friday after hitting a record $6.52 (£4.92) on 22 September.
Mr Trump has recently pressured G7 allies to release emergency reserves and expanded access to tax-exempt red-dyed diesel, which is typically reserved for farm equipment.
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