Tehran is "sowing violence and destruction," an Israeli official said.

The Islamic Republic "is not only Israel's problem, but it is the world's problem. Its behaviour threatens the freedom of navigation and global commerce", the official added.

US Navy forces came to the aid of the crew in response to an emergency distress call and saw evidence of the attack, said an American military statement.

It added that initial indications "clearly point" to a drone-style attack, and that US Navy ships were now escorting the vessel with US personnel aboard to help.

Analysts said the attack bore all the hallmarks of tit-for-tat exchanges in the "shadow war" between Israel and Iran, in which vessels linked to each nation have been targeted in waters around the Gulf.

The Israeli official warned that "our campaign against them (Iran) will continue".

Zodiac Maritime, owned by Israeli billionaire Eyal Ofer, said the incident on board the MT Mercer Street on Thursday killed one Romanian and a UK national, who was a guard for British maritime security firm Ambrey.

The vessel was in the northern Indian Ocean, travelling from Dar es Salaam to Fujairah with no cargo onboard when the attack occurred, it said.

"We are not aware of harm to any other personnel," it said in a statement, adding that the Japanese-owned tanker was back under the control of its crew and was steaming to an undisclosed "safe location" under US naval escort.

Oman's state news agency said the country's navy dispatched a ship and confirmed the attack took place outside the sultanate's territorial waters.

- 'Probably Iran' -

The United States, a key ally of Israel and arch-rival of Iran, expressed concern and said it was working to "establish the facts".

Meir Javedanfar, an expert on Iranian diplomacy and security at Israel's IDC Herzliya university, told AFP the attack was "most probably Iran".

Iran's state TV channel in Arabic Al-Alam, citing "informed regional sources", said the attack was a "response to a recent Israeli attack" targeting an airport in central Syria. It did not provide further details.

Javedanfar said Iranians "feel badly disadvantaged when it comes to responding to attacks inside Iran which have been associated to Israel", including an April strike on the Natanz uranium enrichment site reportedly executed by Israel.

An attack on a maritime vessel "is one area where (Iranians) feel they can try to at least retaliate," he added, calling the latest strike an escalation in the "shadow war" between the two Middle Eastern powerhouses.

But he assessed the fundamental dynamics of the rivalry would change little.

"Both sides will continue what they're doing," he said.

- 'Significant escalation' -

Zodiac Maritime is part of the Zodiac Group, owned by billionaire Ofer, whose enterprises span shipping, real estate, technology, banking and investments.

Ofer was ranked the world's 197th richest person by Forbes this year, with a fortune of $11.3 billion. His firms own and operate over 160 ships.

Zodiac initially called the attack on the MT Mercer Street "a suspected piracy incident".

The Arabian Sea and surrounding Indian Ocean were plagued by piracy around a decade ago, but incidents have waned in recent years after foreign navies stepped up patrols.

An anti-piracy taskforce run by the British Royal Navy, which issued a report of "a vessel being attacked" around 152 nautical miles (280 kilometres) off the coast of Oman, classed the incident as "non-piracy".

Maritime industry analysts Dryad Global said the attack was similar to previous incidents against vessels associated with Israel and Iran.

Two ships operated by Israeli firm Ray Shipping were attacked earlier this year.

"The attack on the MT Mercer Street is now assessed to be the fifth attack against a vessel connected to Israel," Dryad said in an email note on the incident.

But it said before the deaths were confirmed that the loss of two personnel "would represent a significant escalation in events that... would likely lead to significant international condemnation and would require diplomatic redress".

bur-dac/jm/reb

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