The UK and US have blamed Iran for an attack on an oil tanker in which two crew members, including a British national, were killed.
Foreign Secretary Dominic Raab accused Iran of carrying out a “deliberate, targeted” assault which he says constituted “a clear violation of international law”.
Meanwhile, US Secretary of State Anthony Blinken said he was considering “next steps” with the UK and other allies, with “an appropriate response… forthcoming”.
The incident occurred last Thursday when the MV Mercer Street was off Oman’s Coast in the Arabian Sea.
“The UK condemns the unlawful and callous attack committed on a merchant vessel off the coast of Oman, which killed a British and a Romanian national,” Mr Raab said.
“Our thoughts are with the friends and family of those killed in the incident.
“We believe this attack was deliberate, targeted, and a clear violation of international law by Iran.
“UK assessments have concluded that it is highly likely that Iran attacked the MV Mercer Street in international waters off Oman on July 29 using one or more unmanned aerial vehicles (UAVs).
“Iran must end such attacks, and vessels must be allowed to navigate freely in accordance with international law.
“The UK is working with our international partners on a concerted response to this unacceptable attack.”
Mr Blinken said: “Upon review of the available information, we are confident that Iran conducted this attack, which killed two innocent people, using one-way explosive UAVs, a lethal capability it is increasingly employing throughout the region.”
He added: “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. We once again offer our condolences to the families of the victims.”
Earlier this week, Israeli foreign minister Yair Lapid blamed “Iranian terrorism” for the attack.
Iran has not yet commented on the allegations.
A Romanian citizen was also killed in the attack.
Lisa Nandy, Labour’s shadow foreign secretary, said the UK government needs to take a hard line with Tehran.
She said: “The prime minister must make it clear to the incoming Iranian president that lawless actions will carry costs. This is the moment where Britain must show we are resolute in our determination to end this pattern of behaviour.
“The breakdown of a clear strategy to deal with Iran has not served the UK or our allies well in recent years. The foreign secretary must now make it a priority to pursue coordinated international efforts to tackle these actions by the Iranian government.”
The tanker is operated by London-based company Zodiac Maritime, who say they are working to establish what happened.
The incident has sparked concern that tensions are increasing in the region.
A UK source said crew members reported being targeted by “some sort of drone” on Thursday in the Arabian Sea before communications with the ship were lost.
If a drone attack is confirmed it would raise speculation about a possible link to a government or some kind of proxy group.
Iran in the past has repeatedly been accused of targeting tankers in the Gulf.
The country has been locked in a tit-for-tat contest with Israel, with both sides accusing the other of targeting ships and launching cyber attacks.
According to Eikon’s ship tracking, the Mercer Street was headed to Fujairah, a bunkering port and oil terminal in the United Arab Emirates, from Dar es Salaam, in Tanzania.