The United States on Sunday (August 1) joined Britain and Israel in blaming Iran for a deadly tanker attack off Oman, despite Tehran's denials, and Washington vowed an "appropriate response".

The MT Mercer Street, managed by prominent Israeli billionaire Eyal Ofer, was attacked Thursday near the island of Masirah in the Arabian Sea.

A British security guard and a Romanian crew member were killed in what the United States, Britain and the vessel's operator Zodiac Maritime said appeared to be a drone strike.

British Foreign Secretary Dominic Raab said London believed the attack was "deliberate, targeted, and a clear violation of international law by Iran", and "UK assessments" showed Iran used "one or more UAVs" (Unmanned Aerial Vehicles) to target the tanker.

"Iran must end such attacks, and vessels must be allowed to navigate freely in accordance with international law," Raab said in a statement.

US Secretary of State Antony Blinken said Washington was "confident that Iran conducted this attack", and vowed that an "appropriate response" was forthcoming.

Blinken said there was "no justification for this attack, which follows a pattern of attacks and other belligerent behaviour".

"These actions threaten freedom of navigation through this crucial waterway, international shipping and commerce, and the lives of those on the vessels involved."

On Monday, both Romania and Britain summoned their Iran ambassadors.

"Iran must immediately cease actions that risk international peace and security," a British government statement said, adding that "vessels must be allowed to navigate freely in accordance with international law".

"Romania strongly condemns t/Iranian drone attack against 'Mercer Street', during which a RO citizen was killed. There is no justification whatsoever 4 deliberately attacking civilians," Romanian Foreign Minister Bogdan Aurescu tweeted Monday.

Attack raises tensions with Iran

The strike on the Mercer Street marked the first known fatal attack after years of assaults, linked to tensions with Iran over its nuclear activities, on commercial shipping in the region.

The Mercer Street is a Japanese-owned tanker flying a Liberian flag and operating with a foreign crew. London-based Zodiac Maritime, a company owned by Ofer, manages the ship.

No one has claimed responsibility for the attack, and Iran has denied having a role.

However, Iran and its proxy militias have previously used so-called "suicide drones", which crash into targets and detonate their explosive payloads.

Maritime analysts Dryad Global said Thursday's attack was the fifth against a ship connected to Israel since February.

The drone attack blasted a hole through the top of the oil tanker's bridge, where the captain and crew command the vessel, a US official said.

The tanker was travelling from Dar es Salaam in Tanzania to Fujairah in the United Arab Emirates with no cargo aboard when it was hit, Zodiac Maritime said.

The American nuclear-powered aircraft carrier USS Ronald Reagan and the guided missile destroyer USS Mitscher escorted the Mercer Street to a safe port, the US Navy said.

The two fatalities have raised tensions in the Gulf at a critical juncture, with Iran preparing to inaugurate a new president and with international talks to revive the 2015 nuclear deal stalled.