The US military said it would block shipping traffic in and out of Iran’s ports starting at 10am ET (10pm, Singapore time) on Monday (Apr 13), a move that would prevent roughly two million barrels of Iranian oil a day from entering the world’s markets, further tightening global supply.

Here’s how it would work, what it means for oil markets and why US President Donald Trump is threatening to restrict access to a strait that he wants Iran to reopen.

WHAT WAS ANNOUNCED?

After weekend peace talks in Islamabad between negotiators from the US and Iran ended without a deal, Trump said the US Navy “will begin the process of blockading any and all ships trying to enter, or leave, the Strait of Hormuz”.

The US military’s Central Command later said the blockade would apply only to ships going to or from Iran, including all Iranian ports on the Gulf and Gulf of Oman. 

US forces would not impede freedom of navigation for vessels transiting the Strait of Hormuz to and from non-Iranian ports, and additional information would be provided, it said.

Iran’s Revolutionary Guards responded to Trump by warning that military vessels approaching the strait would be considered a ceasefire breach and dealt with harshly and decisively.

Retired Admiral Gary Roughead, a former chief of US naval operations, cautioned that Iran could fire on ships in the Gulf or attack the infrastructure of Gulf states that host US forces.

WHAT IS THE IMPLICATION FOR IRANIAN OIL FLOWS?

Blocking Iranian shipments would disconnect a significant source of oil from the world’s markets. 

Iran exported 1.84 million barrels per day (bpd) of crude in March. It has shipped 1.71 million bpd thus far in April, compared with a full-year average of 1.68 million bpd in 2025, according to Kpler data.

However, a surge in Iranian output before the war started on Feb 28 has led to near-record levels of Iranian oil loaded on ships, with more than 180 million barrels floating as of earlier this month, according to Kpler data.

Before the war, most Iranian oil exports were shipped to China, the top global crude importer. Last month, the US unveiled a sanctions waiver that has enabled other buyers, including India, to import Iranian oil.

India is set to receive its first crude shipment from Iran in seven years this week, ship tracking data from LSEG and Kpler showed on Wednesday.