Discussions between Britain and the United States on building up their military presence in the Gulf are ongoing, Prime Minister Theresa May’s spokeswoman, Alison Donnelly, said on Friday amid tensions with Iran.

Britain also said it was sending a second warship to the Gulf and raising the alert level in the region as tensions spike after Iranian gunboats threatened a UK supertanker.

Relations between Tehran and the West have been increasingly strained after Britain seized an Iranian tanker in Gibraltar and London said the British Heritage, operated by oil company BP, had been approached in the strait between Iran and the Arabian Gulf.

“We are talking to the US about building on our presence in the face of recent threats to shipping in the area,” May’s spokeswoman said.

“The US has been discussing with a number of countries, including ourselves, how we might build on this. Those discussions are currently ongoing,” Alison said.

Bahrain said on Friday it “strongly condemned” what it called an Iranian attempt to intercept a British tanker in the Gulf, according to a statement from its foreign ministry.

“This hostile action embodies Iran’s insistence on threatening security and peace, and harming maritime navigation,” the statement said.

The decision was disclosed as US President Donald Trump stepped up his war of words with the Islamic republic over its nuclear programme and its alleged support for terror groups in the Middle East.

“Iran better be careful,” Trump told reporters outside the White House.

“They’re treading on very dangerous territory. Iran, if you’re listening, you better be careful.”

A source said the British government decided at the start of the week to raise the alert level for ships travelling through Iranian waters to three on a three-point scale.

The decision means that Britain views the security threat in Iranian waters for commercial ships as “critical.”

But UK Foreign Secretary Jeremy Hunt called on Friday for “cool heads” to prevail as threats mount and Iran’s response to its growing isolation — and economic suffering from US sanctions — becomes increasingly unpredictable.

“We want to do everything we can to make sure that we don’t have an unintended escalation which could be very dangerous for the world,” Hunt said.

Shipping companies sailing through the Middle East Gulf are being urged to avoid having private armed security guards onboard as the risk of escalation in the region rises, industry associations say.

British officials said the naval deployment was part of a pre-planned rotation and meant to ensure a continued British naval presence in one of the most important but volatile oil shipment routes in the world. But a source said the switchover was brought forward by several days and should see two of Britain’s most advanced warships jointly navigating the waters off Iran for a period that might last a number of weeks.

A government spokesman said the HMS Duncan was “deploying to the region to ensure we maintain a continuous maritime security presence while HMS Montrose comes off task for pre-planned maintenance and crew changeover.”

Sky News television reported that HMS Duncan would not be part of the international military escort team for vessels in the Gulf proposed on Thursday by the United States.

HMS Duncan is an air defence destroyer that carries a set of heavy Harpoon anti-ship missiles and a crew in excess of 280.

The BBC reported that it had reached Turkey’s Bosphorus Strait as it makes its way from the Black Sea through the Suez Canal to the Gulf.

HMS Montrose was forced to warn off three fast Iranian gunboats on Wednesday that UK officials said were trying to “impede” the progress of a British supertanker through the Strait of Hormuz in the Gulf.

An advisory issued in recent days by leading shipping associations warned against using private armed guards in the critical areas.

“The use of force against threats recently encountered in the Gulf of Oman carries significant risk and has the potential to escalate security situations to the detriment of the safety of ship and crew,” the advisory said.

“The use of unarmed maritime advisors to assist with onboard security and watch-keeping is sensible,” it said, noting relevant legal guidelines.

Agencies