A US military blockade of maritime traffic entering and leaving Iranian ports is due to begin in a matter of hours on Monday.
The US says ships coming or going from elsewhere would be allowed to pass through the Strait of Hormuz, a critical waterway Iran effectively closed in response to US-Israeli strikes.
It comes after negotiators from both sides failed to reach a deal to end the war, which began on 28 February but is currently under a two-week truce.
US President Donald Trump has said direct talks with Iran in Pakistan failed because Iran was "unwilling to give up its nuclear ambitions" - but a US official said there had been a much longer list of disagreements, including on Iran's control of the strait.
Here's what we know about the US announcement.
