Iran tanker seizu
The UK government has said it is "deeply concerned" about Iran"s "unacceptable" seizure of a British-flagged tanker in the Gulf.
The Stena Impero"s owners say they have been unable to contact the ship, which was seized in the Strait of Hormuz, a key waterway in the region.
Foreign Secretary Jeremy Hunt has warned of "serious consequences" if the situation is not resolved quickly.
Iran said the ship was "violating international maritime rules".
A second British-owned Liberian-flagged tanker, the MV Mesdar, was also boarded by armed guards but was released.
What happened?
The Stena Impero was seized on Friday by the Iranian Revolutionary Guard, according to the semi-official Fars news agency.
Mr Hunt said the tanker had been surrounded by four vessels and a helicopter before heading into Iranian waters.
The vessel"s owners said it was fully complying with regulations and was in international waters when it was approached.
It said there no reported injuries among the 23 crew members, who are Indian, Russian, Latvian and Filipino.
A government spokeswoman told the BBC: "We have advised UK shipping to stay out of the area for an interim period."
Mr Hunt said the seizures were "completely unacceptable" and "freedom of navigation must be maintained".
"We are not looking at military options," he added. "We are looking at a diplomatic way to resolve this situation."
What"s the background to this?
The latest developments come amid a deterioration in relations between the UK, the US, and Iran.
Tensions between the US and Iran have risen sharply since April, when the US tightened sanctions it reimposed on Iran after unilaterally withdrawing from a landmark 2015 nuclear deal.
The US has blamed Iran for attacks on tankers which have happened in the world"s key shipping area since May. Tehran denies all the accusations.
Although the UK government remains committed to the deal, which curbs Iran"s nuclear activities in return for the lifting of sanctions tensions escalated further earlier this month when British Royal Marines helped seize an Iranian tanker off Gibraltar.
It was suspected of carrying oil to Syria in breach of EU sanctions.
In response, Iran threatened to seize a British oil tanker.
On Friday, Gibraltar granted a 30-day extension to allow authorities to detain the tanker longer.
Since the seizure of the Iranian tanker, the UK raised the threat to British shipping in Iranian waters in the Gulf to "critical".
Iranian boats also attempted to impede a British oil tanker in the region, before being warned off by a Royal Navy ship, according to the Ministry of Defence. Iran denied any attempted seizure.
On Friday, the US claimed to have destroyed an Iranian drone in the Gulf