Police in Gibraltar have arrested the captain and chief officer of the Iranian supertanker detained last week in an operation involving British Royal Marines.
The move comes just hours after a Royal Navy warship drove off Iranian patrol boats as they attempted to impede the progress of a British tanker through the Strait of Hormuz.
The incidents come a day after Iranian President Hassan Rouhani warned of unspecified “repercussions” for the seizure of the Grace 1 tanker off Gibraltar last Thursday.
In London, ministers urged Tehran to “de-escalate” the situation in the Gulf.
But the arrest of the ship’s officers of the Grace 1 in relation to suspected violations of EU sanctions on Syria is likely to exacerbate the already heightened tensions in the region.
In a statement, the Royal Gibraltar Police said the arrests followed a “protracted” search of the vessel, which remains in detention, during which documents and electronic devices were seized.
Earlier the Ministry of Defence confirmed the Type 23 frigate HMS Montrose had been forced to intervene when three Iranian fast patrols attempted to “impede” the passage of the BP-operated tanker British Heritage.
According to US media reports, a US military aircraft was in in the region and took video footage of the incident in which the Montrose was reported to have trained its guns on the Iranian boats.
A MoD spokesman said: “HMS Montrose was forced to position herself between the Iranian vessels and British Heritage and issue verbal warnings to the Iranian vessels, which then turned away.”
“We are concerned by this action and continue to urge the Iranian authorities to de-escalate the situation in the region.”
The Iranian Revolutionary Guard, which is thought to have been operating the patrol boats, denied the incident, saying if it had received orders to seize any ships it would have done so immediately.
Iranian foreign minister Mohammad Javad Zarif also dismissed the British account as “worthless”, saying the claims were being made to “create tension”.
The clash happened after the Department for Transport this week raised its security level for British shipping sailing in Iranian waters to its highest level, level three, meaning an incident is considered to be imminent.
Vessels were advised to take enhanced security measures and to avoid transiting through Iranian waters if at all possible.
Foreign Secretary Jeremy Hunt described the developments as “very concerning” and said security in the region was kept under constant review.
“We are constantly monitoring security and constantly keeping under review the kind of security we need to keep British shipping safe,” he said.
As well as the Montrose, the Navy has four mine countermeasures vessels and the Royal Fleet Auxiliary Cardigan Bay logistics ship in the region.
With typically between 15 and 30 British-flagged merchant ships in Gulf waters on any one day, providing individual escorts has been ruled out.
The latest incidents come at a particularly sensitive time as tensions between the US and Iran have been ratcheting up over the unravelling of the Iran nuclear deal, which President Donald Trump withdrew from last year.
The European parties to the agreement – including the UK – earlier this week expressed “deep concern that Iran had begun enriching uranium to a higher level of purity than permitted under the terms of the agreement”.
Last month, Mr Trump said he had made a last-minute decision to call off air strikes in retaliation for the shooting down by Iran of an unmanned US drone.
Meanwhile, the US has said it will move ahead with plans to build a coalition of nations to monitor and deter Iranian threats against commercial shipping in the Gulf.
Marine General Joseph Dunford, the chairman of the US Joint Chiefs of Staff, said the Pentagon has developed a specific plan and it would become clear within a couple of weeks which nations were willing to join the effort.
He said: “We’re getting ready now to move out. We have a pretty clear concept of what we want to do.”