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.