Six senators are calling on the secretaries of State and Treasury to review whether a potential Russian takeover of petroleum company Citgo could threaten national security and violate economic sanctions.

Citgo, owned by the government of Venezuela, risks defaulting on a loan from Rosneft, a Russian state-owned energy company. Rosneft could take control of Citgo upon default, giving a company with close ties to Russian President Vladimir Putin command of United States infrastructure.

The senators, including Senate Finance Committee ranking Democrat Ron Wyden (Ore.), Foreign Relations Committee ranking Democrat Ben Cardin (Md.) and Majority Whip John Cornyn (R-Texas), expressed national security concerns in a Monday letter to Secretary of State Rex Tillerson and Treasury Secretary Steven Mnuchin.

Sens. Ted Cruz (R-Texas), Marco Rubio (R-Fla.) and Richard Blumenthal (D-Conn.) also signed the letter.

"Citgo operates across 19 states, with 48 terminals, interstate oil and gas pipelines, and refineries," the senators wrote. "Serious questions have been raised regarding the national security risk of Rosneft - a company with close ties to President Putin - assuming control of U.S. energy infrastructure."

The senators also asked Tillerson and Mnuchin to report on whether Rosneft's acquisition of Citgo's U.S.-based assets would violate economic sanctions against Russia.

Rosneft has been subject to U.S. sanctions since Russia's annexation of Crimea from Ukraine in 2014. The company's executive chairman, Igor Sechin, has also been blocked from the U.S. financial system.

"The Treasury Department must immediately determine the likelihood of Rosneft acquiring a controlling stake in Citgo," the senators wrote. "Reports suggest that a license could be required for Citgo to continue orderly operations should Rosneft take control. If that is the case, it would appear that, barring the license, the transaction would necessarily result in one or more sanctions violations."