At least three container ships were hit by gunfire in the Strait of Hormuz on Wednesday, maritime security sources and the United Kingdom Maritime Trade Operations said.

Iran has imposed restrictions on ships using the strait, first in retaliation for the US-Israeli bombardment of the country and then in response to a US blockade of Iranian ports.

A Liberia-flagged container ship sustained damage to its bridge after being hit by gunfire and rocket-propelled grenades northeast of Oman.

The United Kingdom Maritime Trade Operations (UKMTO) said the master of the vessel reported being approached by an Islamic Revolutionary Guard Corps (IRGC) gunboat. The vessel, it said, was subsequently fired upon. All crew members were safe and there was no fire or environmental impact due to the incident.

Maritime security sources told Reuters that three people were onboard that gunboat. The master of the Greek-operated container ship also reported that no radio contact was made prior to the incident and that the vessel had been initially informed that it had permission to transit the Strait of Hormuz.

The UKMTO later said that a second container vessel had been fired upon about eight nautical miles west of Iran. The Panama-flagged vessel was not damaged and its crew members are safe.

Maritime security sources told Reuters that a third container ship was fired upon about eight nautical miles west of Iran while transiting through the Strait of Hormuz. The Liberia-flagged vessel, which was not damaged, had stopped in the water. Its crew are safe, the sources said.

The attacks came days after the US seized an Iranian container ship after shooting it this past weekend and boarded an oil tanker associated with Iran's oil trade in the Indian Ocean.

Late on Tuesday, US President Donald Trump said the US would indefinitely extend the ceasefire with Iran, which had been due to expire on Wednesday, to give Tehran time to come up with a "unified proposal" ahead of possible negotiations.

Iran has offered no formal acknowledgment of Trump's ceasefire extension.

Pakistan's Prime Minister Shehbaz Sharif thanked Trump for agreeing to the ceasefire extension, saying it would buy time for ongoing diplomatic efforts.

"With the trust and confidence reposed in us, Pakistan shall continue its earnest efforts for a negotiated settlement of the conflict," he wrote on X.

Trump said the US would continue its blockade of Iranian ports, which Iran has called "unacceptable," and has indicated was a reason it had not yet agreed to join talks in Islamabad.

The Revolutionary Guard on Wednesday stressed the need for vigilance during a “silent battlefield” and for monitoring enemy actions during “the so-called ceasefire.”

The Guard said it was ready to confront any new aggression and would “inflict crushing blows beyond the enemy's imagination to its remaining assets in the region.”

In peacetime, about 20 per cent of the world's oil and natural gas transits the Strait of Hormuz, which leads from the Persian Gulf to the open oceans and was fully open until the US and Israel attacked Iran on February 28 to start the war.

Since then Tehran has throttled shipping traffic through the strait, causing oil prices to skyrocket and impacting global economies.

In early trading on Wednesday, Brent crude oil, the international standard, was trading at close to $98 a barrel, up more than 30 per cent since the day the war started.

Pakistan has been working to bring both sides together for a second round of talks.

So far, Iran has not committed but Pakistani officials there have expressed confidence that Tehran will send a delegation to resume negotiations. The first round on April 11 and 12 ended without an agreement.

Over the weekend, Iran said that it had received new proposals from Washington, but also suggested that a wide gap remains between the sides. Issues that derailed the previous round of negotiations included Iran's nuclear enrichment program, its regional proxies and the strait.

Following Trump's announcement of the ceasefire extension, UN Secretary-General Antonio Guterres said he hoped it would create "critical space for diplomacy and confidence-building between Iran and the United States," according to his spokesman, Stephane Dujarric.