Iran says it seized a “foreign tanker” and its 12 crew on Sunday for smuggling fuel in the Gulf.
Iran’s state media published footage of Iranian speedboats circling around the Panamanian-flagged Riah tanker.
In a separate development, US President Donald Trump said the US Navy downed an Iranian drone in the Strait of Hormuz.
He said the USS Boxer “took defensive action” after the drone had come within about 1,000 yards (914m) of the vessel.
Iran has so far made no public comments on the drone.
Relations between the US and Iran are badly strained, with the US blaming Iran for attacks on oil tankers in the Gulf. Iran denies the allegations.
What did Iran say about the tanker?
The Revolutionary Guards’ Sepah News site said on Thursday that the alleged fuel-smuggling ship was seized on Sunday during naval patrols aimed at “discovering and confronting organised smuggling”.
The ship had a two-million-litre capacity, but was carrying one million litres of fuel when it was seized south of Iran’s Larak Island, the website said.
The ship and crew were caught “trying to hand over smuggled fuel which it had received from Iranian dhows to foreign ships further afield”, it went on.
The case is now “going through judicial procedures”.
State media later published footage of Iranian speedboats circling around the Panamanian-flagged Riah tanker.
Iran earlier in the week said it had come to the aid of a broken-down tanker, without naming the Riah. It made no mention of seizing the ship.
The UK government said it was seeking further information over the reports and urged the Iranian authorities to “de-escalate the situation in the region”.
What’s the background to this?
Tensions have been high in the Gulf since the US tightened the sanctions that it reimposed on Iran’s oil sector after unilaterally withdrawing from a landmark 2015 nuclear deal.
The US has blamed Iran for two separate attacks on oil tankers in the Gulf of Oman in May and June – an allegation Tehran has denied.
Iran also shot down a US surveillance drone over the Strait of Hormuz in disputed circumstances.
UK warships have meanwhile been shadowing British oil tankers in the area since Iran threatened to seize one in response to the impounding of an Iranian tanker off Gibraltar.
The UK said the tanker was suspected of breaching EU sanctions against Syria. Iran denied it was en route there.
US Central Command Chief General Kenneth McKenzie, speaking on a visit to Saudi Arabia on Thursday, said he was working “aggressively” to find a solution to free passage for ships through the Gulf region, Reuters news agency reports.
The situation of missing or impounded tankers in the congested waters of the southern Gulf is far from clear.
Commercial vessels navigating the Gulf, the Strait of Hormuz and the high seas are all normally equipped with a transponder known as AIS (Automatic Identification System).
This transmits their location on two dedicated VHF (Very High Frequency) channels.
When it is switched off, the ship’s signal disappears from the satellite map, making it difficult, as in this case, to work out where a ship has gone to and what it is up to.
What is beyond doubt is that illegal smuggling has been going on for decades in the Gulf, including by oil tanker captains hugging the Iranian coastline to stay out of international waters, allegedly bribing local coastguards along the way.
For years a fleet of fast Iranian speedboats has plied the short distance between the Omani port of Khasab and Iranian ports close to Bandar Abbas, evading coastguard patrols and smuggling cigarettes and other lucrative contraband.
Gulf tensions: Recent events
May 2018: US President Donald Trump withdraws from the 2015 nuclear deal between Iran and six world powers, and later imposes punishing sanctions against Iran
2 May 2019: Mr Trump steps up pressure on Tehran by ending exemptions from secondary sanctions for countries still buying Iranian oil
5 May: The US sends an aircraft carrier strike group and B-52 bombers to the Gulf because of “troubling and escalatory indications” related to Iran
8 May: Iranian President Hassan Rouhani says Iran will scale back its commitments under the nuclear deal in retaliation for the US sanctions
12 May: Four oil tankers are damaged by explosions off the UAE coast in the Gulf of Oman. The UAE says the blasts were caused by limpet mines planted by a “state actor”. The US blames Iran, but Tehran denies the allegation
13 June: Explosions hit two oil tankers in the Gulf of Oman. The US again accuses Iran, releasing footage purportedly showing Iranian forces removing an unexploded limpet mine from a damaged vessel. Iran says the evidence is fabricated
20 June: Iranian forces shoot down a US military drone over the Strait of Hormuz
1 July: Iran breaches a limit on its stockpile of low-enriched uranium set under the nuclear deal. Enriched uranium is used to make reactor fuel, and potentially nuclear weapons
4 July: Gibraltar, assisted by British Royal Marines, seizes Iranian oil tanker Grace 1 on suspicion of breaking EU sanctions on trade with Syria. Iran says the tanker was not going to Syria
7 July: Iran breaches another commitment under the nuclear deal by resuming enriching uranium to higher levels. It vows to reverse the step if European countries do more to mitigate the effects of the US sanctions
10 July: UK says Iranian naval vessels tried to impede a British tanker as it moved into the Strait of Hormuz, before being forced to back off by a Royal Navy frigate. Iran denied attempting to seize the tanker.