The Treasury Department determined that the National Iranian Oil Company is "an agent or affiliate" of the guard corps (IRGC), which the United States has long sanctioned for terrorism and human rights abuses.

Congress directed Treasury to determine whether Iran's oil and tanker companies were linked to the IRGC as part of a new package of sanctions signed into law in August.

Adam Szubin, the head of Treasury's Office of Foreign Assets Control, told Congress that the IRGC's influence has grown in the oil company and Iran's energy sector.

But Szubin said there was not enough information at this time to conclude that the National Iranian Tanker Company was also linked to the IRGC.

U.S. backers of sanctions applauded the determination as Washington tightens sanctions over Iran's oil sales to cut funding to its nuclear program that the West believes is to make bombs. Tehran says the program is purely for civilian purposes.

"Oil transactions with NIOC directly support the IRGC's role in Iran's nuclear-weapons program and its support for terrorism," Representative Howard Berman, the top Democrat on the House Foreign Affairs Committee, said in a statement.

Senator Robert Menendez, a Democrat who helped craft the sanctions law, urged the Obama administration to continue investigating whether Iran's elite guards are linked to the tanker company.