White House bureau chief Steve Herman contributed to this report.
Iran says it has seized a British oil tanker that was passing through the Strait of Hormuz.
Iran's state television said Friday that Iran's Revolutionary Guard Corps captured the tanker, Stena Impero, off the Iranian province of Hormozgan for "not following international maritime regulations."
It said the tanker was taken to a coastal area and turned over to authorities.
Reuters news agency reports the company that manages the vessel, Northern Marine Management, was not able to contact the crew of the tanker after the vessel was approached by unidentified small crafts and a helicopter in the Strait of Hormuz. The company said the vessel was now heading north toward Iran.
U.S. National Security Council spokesman Garrett Marquis said, "We are aware of reports that Iranian forces seized a British oil tanker," and said the United States "will continue to work with our allies and partners to defend our security and interests against Iran's malign behavior."
Iran has already been accused by Washington of sabotaging a half-dozen oil tankers in the Strait of Hormuz. Iran has denied it has mounted any mining operations against shipping in the strait.
Earlier this month, Britain detained an Iranian tanker on allegations that it was headed to Syria in violation of sanctions. That detention sparked outrage in Tehran, which accused London of doing the bidding of Washington in action that is "tantamount to maritime banditry."
The Strait of Hormuz is a narrow sea passage between the Persian Gulf and the Gulf of Oman and one of the world's most important oil arteries. The 39-kilometer-wide and 154-kilometer-long strait is the only sea route to the open ocean for more than one-sixth of global oil production and a third of the world's liquified natural gas.