The UK government has said it is "deeply concerned" about Iran's "unacceptable" seizure of a British-flagged tanker in the Gulf, reports BBC.
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.
The Stena Impero was seized on Friday by the Iranian Revolutionary Guard, according to the semi-official Fars news agency.
Stena Bulk, which owns the vessel, said it was in "full compliance with all navigation and international regulations" and was approached while it was in international waters.
Mr Hunt said the tanker had been surrounded by four vessels and a helicopter and was heading into Iranian waters.
There were no reported injuries among the 23 crew members, who are Indian, Russian, Latvian and Filipino, Stena Bulk said.
Norbulk Shipping UK, the Glasgow-based operator of the MV Mesdar, said communication had been re-established with the vessel and its crew was "safe and well".
The government's emergency committee, Cobra, met in Whitehall twice on Friday in response to the crisis.
A spokeswoman told the BBC the government was "deeply concerned about Iran's unacceptable actions", saying they "represent a clear challenge to international freedom of navigation".
"We have advised UK shipping to stay out of the area for an interim period," the spokeswoman added.
Earlier, Mr Hunt, who attended the Cobra meeting, said the seizures were "completely unacceptable" and "freedom of navigation must be maintained".
"We are absolutely clear that if this situation is not resolved quickly there will be serious consequences," he said.
"We are not looking at military options. We are looking at a diplomatic way to resolve this situation but we are very clear that it must be resolved."
Labour leader Jeremy Corbyn said: "The seizure of these vessels is unacceptable, and the tanker that remains under Iranian control must be released. Escalation risks a slide into an even deeper conflict."
US President Donald Trump responded to the developments by saying he would talk to the UK.
And US Central Command - which is responsible for Washington's security interests across the Middle East - said it was developing a multinational maritime effort in response to the situation.
Operation Sentinel will increase surveillance of, and security in, key waterways in the Middle East to ensure freedom of navigation, it said.
The US military said it wanted to promote maritime stability, ensure safe passage, and de-escalate tensions in international waters throughout the Arabian Gulf, Strait of Hormuz, the Bab el-Mandeb Strait and the Gulf of Oman.