Iranian forces boarded a tanker in international waters, using a helicopter and two ships to take over the vessel, the US Central Command said in a tweet on Wednesday.
"Today in international waters, Iranian forces, including two ships and an Iranian "Sea King" helicopter, overtook and boarded a ship called the 'Wila'," Centcom said.
It also posted grainy black-and-white footage of a helicopter hovering low over a vessel, and what appeared to be two personnel rappelling onto the deck.
The ship was most recently positioned near the Strait of Hormuz — a chokepoint for a third of the world's seaborne oil — after floating off the eastern coast of the United Arab Emirates for about a month, Bloomberg said citing its tracking data.
It said the vessel is a Liberian-flagged chemical and oil products tanker, and that it had stopped in July near the Iraqi oil terminal of Al-Basra where its draft increased, indicating it had picked up a shipment.
Iran and its arch enemy the United States have traded barbs in the past year over a spate of incidents in the sensitive waters of the Gulf.
The escalation of Iran-US tensions last year saw ships mysteriously attacked, drones downed and oil tankers seized in the strait.
In July 2019, Iran's Revolutionary Guards seized the British-flagged oil tanker Stena Impero in the waterway for allegedly ramming a fishing boat and released it two months later.
The Guards seized at least six other ships last year over alleged fuel smuggling.
Tensions have escalated since 2018 when Trump withdrew the United States from a multinational accord that froze Iran's nuclear programme, and reimposed crippling sanctions on its economy.