UPDATED 8.30pm
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.
Both Israel and the United States denounced the seizure as piracy, with Israel also demanding 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, Israeli 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.
In Washington, National Security Council spokeswoman Adrienne Watson called on Iran "to release the vessel and its international crew immediately".
"Seizing a civilian vessel without provocation is a blatant violation of international law, and an act of piracy", she said.
President Joe Biden cut short a weekend trip to his home state of Delaware for urgent consultations in Washington on the Middle East, the White House said.
Portuguese flag
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 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.
Biden said on Friday that he expected Iran to strike Israel, which was widely blamed for the consulate attack, "sooner rather than later".
Analyst Karim Bitar said he doubted Saturday's ship seizure alone amounted to the anticipated Iranian retaliation.
"After the US warnings... if what happened turns out to be Iran's response, it would appear as anti-climactic," said Bitar, associate researcher at the Institute for International and Strategic Affairs in Paris.
"So, either this is not part of Iran's response or it is a first step towards a wider escalation," he said.
"It would mean that the Iranians are going to progressively test the limits of the United States and of the Israelis."
The surging tensions come against the backdrop of Israel's war against Palestinian militant group Hamas in Gaza, now in its seventh month.
The conflict has drawn in regional actors including Iran-backed groups in Yemen, Lebanon, Syria and Iraq.
Yemen's Iran-backed Huthi rebels have carried out dozens of missile and drone attacks against shipping in the Red Sea, and in November seized an Israel-linked cargo vessel, the Galaxy Leader. The crew are still being held.
Iran has supported Hamas, but repeatedly denied direct involvement in the October 7 attack on Israel that started the war.
In January, it seized a ship off the coast of Oman in retaliation for what it described as the "theft" by the United States of its oil.
Iran's navy had boarded the Greek-owned St Nikolas before later releasing the crew members.