The US military blockades all Iranian ports from 1400 GMT today after the collapse of peace talks in Pakistan.
US Central Command confirms the move follows the collapse of ceasefire talks in Pakistan on Sunday.
President Donald Trump blames Iran for refusing to drop its nuclear programme.
The blockade targets vessels that enter or leave Iranian ports and coastal areas in the Arabian Gulf and the Gulf of Oman.
US forces do not block ships that transit the Strait of Hormuz to or from non-Iranian ports.
Trump announced the operation on his Truth Social platform on Sunday.
He states that the US Navy will stop ships from entering or leaving the strait and will respond if Iran fires on vessels.
Pakistan continues to mediate between the sides and calls on both to respect the two-week ceasefire agreed last week.
Negotiators left Islamabad without a deal.
Oil prices rise by eight percent on Monday and both WTI and Brent contracts top 100 dollars a barrel.
The war began in late February when Israel and the US struck Iran and Iran hit back at targets in the Gulf and Israel.
The conflict kills thousands and disrupts the global economy.
US officials say Iran must not keep a nuclear programme.
Trump states that Iran will never have a nuclear weapon.
Vice-President JD Vance tells reporters that Washington made its final offer.
Iran’s parliament speaker Mohammad Bagher Ghalibaf says Tehran will not bow to threats from Washington.
The country’s navy chief calls the blockade plan unworkable.
Hezbollah terrorists launch rockets towards towns in northern Israel overnight.
The group continues attacks that started in early March after the death of Iran’s supreme leader in the first strikes.
Israeli forces strike Beirut and other areas in Lebanon last week and kill hundreds according to Lebanese health authorities.
Lebanese and Israeli officials plan talks in Washington on Tuesday.
The US military says two of its warships entered the strait on Saturday to clear mines.
Iran denies the claim and states that two Pakistan-flagged oil tankers turned back before reaching the area.
Experts note that the blockade marks a return to open conflict after the temporary pause.
