Iran's Revolutionary Guards seized a container ship "related to the Zionist regime (Israel)" in the Gulf on Saturday, state media reported, with tensions soaring in the region.

The ship's operator, the Italian-Swiss group MSC, later confirmed Iranian authorities had boarded it.

Israel called it "a pirate operation" and demanded that the Guards be declared a "terrorist organisation" by the European Union.

A container ship "was seized by the Sepah (Guards) Navy Special Forces by carrying out a heliborne operation," state news agency IRNA reported, naming it as the MSC Aries.

It added that the operation took place "near the Strait of Hormuz" and "this ship has now been directed towards the territorial waters" of Iran.

MSC confirmed the Aries had "been boarded by Iranian authorities via helicopter as she passed the Strait of Hormuz" on Saturday morning.

It said that 25 crew were onboard and that it was "working closely with the relevant authorities to ensure their wellbeing, and safe return of the vessel."

The Strait of Hormuz connects the Gulf with the Indian Ocean and, according to the US Energy Information Administration, more than a fifth of annual global oil consumption passes through it each year.

A video shared on social media appeared to show people descending from a helicopter onto the deck of the Aries using a rope.

Responding to the seizure, Israel's foreign minister Israel Katz said in a post on X that Iran had conducted "a pirate operation in violation of international law."

"I call on the European Union and the free world to immediately declare the Iranian Revolutionary Guards corps as a terrorist organisation and to sanction Iran now," he added.

Vessel tracking websites, vesselfinder.com and marinetraffic.com, say MSC Aries is a Portuguese-flagged container ship and gave its last reported position as in the Gulf.

The United States said on Friday it was sending military reinforcements to the region, after Iran vowed retaliation for a deadly strike on the consular annexe of its Damascus embassy almost two weeks ago.

The April 1 strike killed at least seven members of Iran's Revolutionary Guards, including two generals. (AFP)