It was the latest incident in the region amid months of heightened tensions between Tehran and the U.S.
The announcement came as the U.S. released new evidence to support its allegation that Iran was responsible for the attack on Saudi Arabia's oil sites.
Iran announced it has begun using advanced centrifuges to enrich uranium.
The incident, involving two foreign ships, comes a day after Iran seized another tanker and the U.S. claimed to have downed an Iranian drone.
U.S. Secretary of State Mike Pompeo is blaming Iran for the destruction of Norwegian-owned and Japanese-owned oil tankers in the Gulf of Oman.
The U.S. says it was a limpet mine attached to the ship in the Gulf of Oman. The Japanese vessel's owner says something flew toward the ship.
The Secretary of State said the attacks are part of a “campaign” of “escalating tension” by Iran.
One of them was reportedly en route to pick up Saudi oil to take to the United States.
U.S. Navy Seals have seized a Libyan tanker that was under the command of Libyan Rebels.
Unchecked exploitation of the British Columbia's priceless coastline has mortgaged our future while accruing a massive ecological debt.