U.S. to Blockade Ships Calling at Iranian Ports on April 13, CENTCOM Says

Mariners in the Gulf of Oman woke up Sunday to new guidance from the U.S. military: stay tuned for a formal notice, and be ready to hail American warships on channel 16 before approaching Iran’s coastline.

Within 24 hours, that advice is set to become part of a sweeping naval operation. U.S. Central Command (CENTCOM) has said American forces “will begin implementing a blockade of all maritime traffic entering and exiting Iranian ports on April 13 at 10 a.m. ET,” a move that marks one of the sharpest escalations yet in the standoff with Tehran and immediately rattled oil markets.

In a statement posted on social media and reported by multiple news outlets, CENTCOM said the operation would target ships of any flag that call at Iran.

“The blockade will be enforced impartially against vessels of all nations entering or departing Iranian ports and coastal areas, including all Iranian ports on the Arabian Gulf and Gulf of Oman,” the command said, according to Yonhap News Agency.

At the same time, the military drew a clear line around the Strait of Hormuz, the narrow waterway that connects the Persian Gulf to global markets and carries a significant share of the world’s seaborne oil.

CENTCOM forces “will not impede freedom of navigation for vessels transiting the Strait of Hormuz to and from non‑Iranian ports,” the statement said. In other words, the United States is signaling that it intends to choke off Iran’s seaborne trade without formally shutting the strait to other countries’ traffic.

CENTCOM told commercial mariners that “additional information will be provided…through a formal notice prior to the start of the blockade,” and advised ships operating near the Gulf of Oman and the Strait of Hormuz to monitor navigational warnings and contact U.S. naval forces over standard bridge‑to‑bridge radio.

The announcement came hours after President Donald Trump offered a broader and more sweeping description of his intentions.

In a post on his Truth Social platform, Trump wrote that the U.S. Navy would “begin the process of BLOCKADING any and all Ships trying to enter, or leave, the Strait of Hormuz” and said he had instructed it to “seek and interdict every vessel in International Waters that has paid a toll to Iran.”

That language, focused on the entire strait and on ships that had paid Iranian fees, goes further than the operational description in CENTCOM’s message, which centers on vessels “entering or departing Iranian ports and coastal areas” and explicitly promises not to interfere with ships traveling to and from non‑Iranian ports.

The gap underscores the difference between political rhetoric and the military’s declared rules for an operation that carries far‑reaching legal and diplomatic consequences.

Under long‑standing law of naval warfare, articulated in instruments such as the nonbinding San Remo Manual, a blockade is generally regarded as a belligerent act. To be lawful, it must be formally declared, made effective in practice, applied impartially to all nations and configured to avoid excessive harm to civilians, including by allowing for humanitarian relief.

By stating that the blockade will be enforced “impartially against vessels of all nations,” CENTCOM signaled that foreign‑flagged shipping, including tankers and cargo ships owned or chartered by U.S. partners, could be stopped if they are bound to or from Iranian ports. That prospect is likely to test Washington’s relations with allies that depend on Gulf energy exports and have so far been cautious about openly joining a U.S.-led campaign against Iran at sea.

The timing of the move is striking. A provisional two‑week ceasefire between the United States and Iran took effect April 8, with Pakistan acting as mediator. As part of that understanding, reporting has indicated that the Strait of Hormuz would reopen for limited passage, offering a brief pause in months of tit‑for‑tat attacks and shipping disruptions.

On April 11 and 12, senior U.S. and Iranian officials met in Islamabad for face‑to‑face talks aimed at extending the truce and addressing nuclear and maritime issues. Those discussions ended without an agreement, and Trump’s blockade announcement followed soon after, framing the naval operation as leverage on Tehran.

Iran has responded with its own warnings. The Islamic Revolutionary Guard Corps (IRGC) said that any military vessels attempting to approach the Strait of Hormuz “will be considered a violation of the ceasefire and will be dealt with harshly and decisively,” according to statements carried by international wire services and Iranian state media.

The IRGC’s naval command added that “the strait is under the control and smart management of Iran’s Navy” and is “open for the safe passage of non‑military vessels in accordance with specific regulations.” Taken together, those comments suggest Tehran is trying to reassure commercial shippers while cautioning foreign navies against operating near its waters.

The risk is that the U.S. plan to intercept traffic linked to Iranian ports in the Persian Gulf and Gulf of Oman will bring American warships and aircraft into exactly the zones Iran says are off‑limits under the current ceasefire, raising the likelihood of encounters that could spiral into direct clashes.

Markets already are reacting to that prospect. Oil prices jumped after reports of the planned blockade, with both U.S. crude and international benchmark Brent rising in early trading, according to the Associated Press. The Gulf region, including traffic through the Strait of Hormuz and surrounding sea lanes, accounted for roughly one‑fifth of global seaborne oil flows before the latest conflict, making it one of the most sensitive chokepoints in the global economy.

Any sustained effort to halt ships bound to or from Iran could tighten effective oil supply by curbing Iranian exports, while also pushing up insurance and freight rates for tankers operating anywhere near the area. Insurers typically impose “war‑risk” premiums when ships enter high‑threat zones, costs that can filter through to fuel prices and, eventually, consumer goods.

Beyond energy markets, the blockade raises questions about humanitarian access to Iran. The San Remo Manual and related legal guidance emphasize that blockading states must allow passage of relief supplies for civilians under certain conditions. CENTCOM’s publicly reported statement does not spell out whether food, medicine or other humanitarian cargoes will be exempt from interception, leaving an important detail to be clarified in the promised notice to mariners.

The United States has carried out large‑scale maritime interdiction operations before, from the “quarantine” of Cuba during the 1962 missile crisis to the enforcement of U.N. sanctions on Iraq and North Korea. But declaring a blockade against all ships entering or leaving an adversary’s ports — and applying it against vessels of third countries — is rare in the post‑Cold War era and likely to be hotly debated in foreign capitals.

In the immediate term, shipping companies, insurers and navies around the world will be watching for the fine print of CENTCOM’s notice ahead of the Monday start time: which coordinates are covered, how close to Iranian territorial waters U.S. forces intend to operate, what procedures they will use to query and board suspect vessels, and whether any categories of cargo are explicitly exempt.

They will also be looking to see who, if anyone, joins the U.S. effort. Some long‑standing American allies have signaled reluctance to participate directly in a blockade, preferring to focus on escorting their own flagged ships or pressing for renewed diplomacy.

How strictly Washington enforces the new measures — and how Tehran chooses to respond — will shape not only the fate of the fragile ceasefire but also the security of a sea lane that underpins global trade and energy supplies.