Iranian state media are reporting that the country's paramilitary Islamic Revolutionary Guards Corps (IRGC) has seized a foreign tanker and crew that it says were carrying illegal oil.
The December 30 reports did not say what country's flag the ship was flying.
But the Iranians said the vessel was carrying more than 1 million liters of fuel and was taken into custody on December 29 near Larak Island in the Strait of Hormuz, according to an IRGC press statement quoted by the semiofficial Fars news agency.
Some reports suggested there were 16 Malaysian crew members aboard the ship, while others said 12 "foreign nationals" were detained with the ship.
There was no immediate confirmation by international shipping companies, shipping authorities, or foreign governments of a seizure.
Iranian naval troops frequently interdict smaller vessels in the Persian Gulf and Strait of Hormuz for alleged smuggling of fuel.
But Iranian forces have also seized several internationally flagged oil tankers this year in what some regard as tit-for-tat interference with global shipping since the United States reimposed tough unilateral sanctions last year to cut off Iran's oil exports and its access to international markets.
Western authorities earlier this year seized and released an Iranian oil tanker that was suspected of trying to deliver oil to Syria in contravention of an international embargo.
U.S. officials had said earlier on December 30 that they were on the lookout for possible reprisals after U.S. air strikes the previous day against five targets in Iraq and Syria controlled by an Iranian-backed militia in Iraq, Kataib Hizbullah.
The United States and Iran have ratcheted up tensions since Washington withdrew last year from a major nuclear deal that traded sanctions relief for curbs on Tehran's nuclear activities and launched what U.S. officials described as a "maximum pressure" approach.
Iran's military has been participating this week in joint naval drills with Chinese and Russian ships in the Indian Ocean and Gulf of Oman in a signal of increased military cooperation among those countries.
Tehran called the drills part of an effort to boost "international commerce security in the region."
The United States, United Kingdom, and other allies have been escorting commercial vessels with increasing frequency since the spate of seizures in or near the Strait of Hormuz began to threaten oil shipments through one of the busiest fuel transit routes in the world.