The United States and Britain said on Sunday they believed Iran carried out an attack on an Israeli-managed petroleum product tanker off the coast of Oman on Thursday that killed a Briton and a Romanian, both pledging to work with partners to respond.

Iran earlier on Sunday denied it was involved in the incident after it was blamed by Israel.

"Upon review of the available information, we are confident that Iran conducted this attack, which killed two innocent people, using one-way explosive UAVs," U.S. Secretary of State Antony Blinken said in a statement, adding there was "no justification" for the attack.

"We are working with our partners to consider our next steps and consulting with governments inside the region and beyond on an appropriate response, which will be forthcoming," Blinken said.

British Foreign Secretary Dominic Raab said earlier in the day that UK assessments had concluded that it was highly likely that Iran had used one or more drones to carry out the "unlawful and callous" attack.

"We believe this attack was deliberate, targeted, and a clear violation of international law by Iran," he said. The UK was working with international partners on a "concerted response," he added.