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.
Exxon signed eight legal documents with Mr Sechin in May 2014 and did not disclose the activities voluntarily, the Treasury Department said. It described it as an "egregious case".
Exxon, citing administration statements, on Thursday said it believed the sanctions targeted Mr Sechin as an individual and not in his professional role at Rosneft.
It said the Treasury Department had been reviewing the issue for several years.
As head of Exxon, Mr Tillerson signed deals with Rosneft, leading Russia to award him the Order of Friendship in 2013. At Exxon, he spoke against sanctions.
But Texas-based Exxon was forced to wind down drilling after sanctions were imposed.
Mr Tillerson's ties to Russia prompted controversy when US President Donald Trump named him as his choice to lead the State Department.
The US in April said it would not waive sanctions to allow Exxon to resume drilling in Russia.
US sanctions exposed Exxon to up to $1bn in losses by the end of 2014, Exxon said.