The United States is warning Iran that it will not tolerate any disruption to shipping in the Straits of Hormuz.
Vice-President Mohammad Reza Rahimi warned on Tuesday that Iran would shut the Strait of Hormuz if the West imposed more sanctions over its nuclear programme.
The Strait of Hormuz links the Gulf to the Indian Ocean. About 40% of the world's tanker-borne oil passes through it.
The US maintains a naval presence in the Gulf, largely to ensure the transport of oil remains open.
A Pentagon spokesman said the strait "is not only important for security and stability in the region, but also is an economic lifeline for countries in the Gulf, including Iran".
US Fifth Fleet spokeswoman Rebecca Rebarich told the BBC the navy would be ready to act if required