Image copyright Getty Images Image caption Gen Dunford at a press conference with President Trump in April

The US says it wants to create an multinational military coalition to safeguard waters around Iran and Yemen.

Gen Joseph Dunford, chairman of the US military's Joint Chiefs of Staff, said he wanted to "ensure freedom of navigation" in the region, which provides essential trade routes.

The US has blamed Iran for attacks on six oil tankers in May and June.

Gen Dunford said the US was talking to a number of countries with the "political will" to support the plans.

The US would provide "command and control" ships, leading surveillance efforts, he said. However, the aim would be for other countries to offer boats to establish patrols nearby, and escort commercial ships carrying their flags through the area.

Gen Dunford said the US would "work directly with the militaries to identify the specific capabilities" each country has to support the initiative.

The straits of Hormuz and Bab al-Mandab are strategically important locations, providing access from the Indian Ocean to the Gulf and the Red Sea respectively.

About a fifth of oil that is consumed globally passes through the Strait of Hormuz, which connects the Indian Ocean with the Gulf, while oil tankers heading from the the Middle East to Europe via the Red Sea pass must through Bab al-Mandab.

Gen Dunford said the size of the initiative depended on the number of countries which decided to take part.

"With a small number of contributors, we can have a small mission," he said. "We'll expand that as the number of nations that are willing to participate identify themselves."

The US already has a substantial naval presence in the region and participates in several multinational naval taskforces carrying out maritime security, counter-terrorism, anti-piracy operations.

The headquarters of the US Navy's 5th Fleet is in Bahrain, and it also has naval facilities in Djibouti and Kuwait.

The UK, which also has a naval base in Bahrain, said it was "continuously monitoring the security situation" in the region and was "committed to maintaining freedom of navigation in accordance with international law".

The Royal Navy Frigate HMS Montrose has been positioned to shadow an oil tanker sailing under an Isle of Man flag through the Strait of Hormuz.

Meanwhile, Iran's President Hassan Rouhani accused the UK of being "the initiator of insecurity at sea" by seizing an Iranian oil tanker that was heading to Syria off Gibraltar last week.

General Dunford's comments raise many questions.

If the threat from Iran - as the Americans insist - is so pressing then why is it that any operation could still be weeks away?

The US seems willing only to provide command and control, along with surveillance. The actual patrolling and escorting of merchant vessels would be done by others. What message of US leadership does this send?

How many other countries are contributing? That's still not clear. General Dunford says the operation is "scaleable"; its size will depend upon who participates.

And what of its scope? The US sees this as a mission that will not only include the Gulf but also waters off Yemen.

With tensions mounting in the region, many governments will be wary of getting involved but will have to balance this against the practical need to safeguard their vessels. Some may see this as a US effort to construct something that looks like a naval coalition ranged against Tehran.

Why is the US so concerned about shipping?

Last month, the US blamed Iranian forces for attacks on two tankers in the Gulf of Oman, just outside the Strait of Hormuz - an allegation that Iran rejected. It followed an attack on four tankers off the coast of the United Arab Emirates in May.

Days after the June attacks, Iranian forces shot down a US surveillance drone they said had violated Iranian airspace over the Strait of Hormuz. The US insisted the drone was flying over international waters.

Media playback is unsupported on your device Media caption Why does the Strait of Hormuz matter?

US President Donald Trump called off strikes planned in response after being told that as many as 150 people might be killed.

Iran's military has said it would close the Strait of Hormuz if, as President Trump hopes, it becomes unable to export oil due to the sanctions the US has reinstated since pulling out of a landmark nuclear deal with Iran.

However, speaking to the BBC, Iran's ambassador to the UN remained defiant.

"We have managed under different situations worse than we have today, and I am sure we can manage again," Majid Takht-Ravanchi said.

Media playback is unsupported on your device Media caption Iran's representative to the UN: Europeans 'are not honouring their commitments' on nuclear deal

In the past two weeks, Iran has breached two commitments on uranium enrichment in response to the sanctions.

Mr Takht-Ravanchi warned that Iran was prepared to take further steps if Europe fails to compensate it for the economic losses it has suffered.

Iran has also been accused of providing military support to Yemen's rebel Houthi movement, which has attacked vessels in the Red Sea belonging to a Saudi-led coalition backing the Yemeni government.

On Monday, the coalition said it had foiled an attack on a commercial ship using an unmanned boat rigged with explosives in the southern Red Sea, and warned of the threat to international trade. The Houthis denied targeting commercial shipping.

US-Iran tension: Recent events

May 2018: US President Donald Trump withdraws unilaterally from the 2015 nuclear deal between Iran and six world powers, and begins reinstating sanctions to force Iran to renegotiate the accord. Iran's economy slumps as they take effect.

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 sanctions, including by allowing its stockpile of low-enriched uranium to increase. Enriched uranium is used to make reactor fuel and potentially nuclear weapons.

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 it 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.

17 June: Iran says it will breach the limit on its stockpile of enriched uranium set under the nuclear deal on 27 June, unless Europe protects Iranian oil sales.

20 June: Iranian forces shoot down US military drone over the Strait of Hormuz.

4 July: Gibraltar, assisted by British Royal Marines, seizes Iranian oil tanker Grace 1 on suspicion of breaking EU sanctions on trade with Syria