US authorities have fined Exxon Mobil $2m (£1.5m) for violating sanctions against Russia while Secretary of State Rex Tillerson was its chief executive.
Exxon dealt with Igor Sechin, the president of Russian oil giant Rosneft, who was blacklisted, the US Treasury Department said.
Exxon has challenged the finding, calling it “fundamentally unfair”.
The US imposed sanctions against Russia in March 2014 as Russia annexed Crimea and tension rose in Ukraine.
In April 2014, the US added Mr Sechin to the list of people blocked under the rules.