The Treasury Department has fined Exxon Mobil $2 million for violating U.S. sanctions against Russian individuals, citing actions the oil major took while Secretary of State Rex Tillerson was still serving as the company's CEO.

Exxon Mobil said on Thursday it had launched a legal challenge to the finding. It charges that Treasury was seeking to retroactively enforce changes to its original guidance and had denied the company its constitutional right to due process.

The Treasury Department said the violation occurred in May 2014, when Exxon subsidiaries signed legal documents with Igor Sechin, the chairman of Russian oil giant Rosneft and a target of U.S. sanctions. The signings occurred shortly after the United States designated Sechin a Specially Designated National, which prohibits U.S. persons from dealing with him.

In a statement on Thursday, Treasury said Exxon failed to recognize warning signs and "demonstrated reckless disregard for U.S. sanctions requirements." The officials called Exxon "a

sophisticated and experienced oil and gas company" and said the company's senior officers knew that Sechin had been listed as a Specially Designated National by the United States.

Treasury rejected Exxon's claim that it believed there was a distinction between Sechin's personal and professional capacities. It said the Ukraine-Related Sanctions Regulation does not spell out any such distinction.

On Thursday, Exxon said Treasury's Office of Foreign Assets Control was seeking to retroactively apply a new interpretation, which the company said is inconsistent with the White House and Treasury's explicit guidance in the early months of 2014.

It pointed to White House fact sheets indicating the administration would target sanctioned Russians' personal assets, and not the companies they manage. It also identified comments by Obama administration officials that indicated companies could continue to do business with Rosneft.

"ExxonMobil followed the clear guidance from the White House and Treasury Department when its representatives signed documents involving ongoing oil and gas activities in Russia with Rosneft — a non-blocked entity — that were countersigned on behalf of Rosneft by CEO Igor Sechin in his official representative capacity," the company said in a statement.

While leading Exxon, Tillerson had criticized U.S. sanctions on Russia, saying they were not effective. The industry has said the measures put U.S. companies at a disadvantage to European competitors. Tillerson retired from Exxon at the end of last year to take the Secretary of State post in the Trump administration.

Exxon and Rosneft entered a joint venture in 2012 to develop offshore reserves in the Arctic Kara Sea and the Black Sea, as well as onshore assets in Siberia. The following year, they advanced their strategic alliance by adding seven more blocks in the Chukchi, Laptev and Kara seas.

Exxon's operations in Russia represent only a fraction of its global oil and gas production, but the Rosneft joint-venture represents a big part of Exxon's potential future production growth.

Those endeavors have been on ice since 2014, when the United States imposed sanctions on Russia for its incursions into Ukraine. Exxon said in a filing after the sanctions were put in place that its "maximum before-tax exposure to loss from these joint ventures" totaled $1 billion through the end of 2015.

Exxon's holdings in Russia's Arctic waters and the Black Sea, which are not producing for the energy giant due to the sanctions, totaled 63.6 million acres. By comparison, Exxon holds 85,000 net acres across its main Russian venture, an offshore project called Sakhalin, which was in place before the sanctions.