Four vessels have crossed the Strait of Hormuz so far on Monday ahead of a US military blockade of Iranian ports set to begin at 14:00 GMT (15:00 BST).

All four vessels are identified as tankers - carrying either oil, gas or chemicals - according to tracking data provided by MarineTraffic.

US Central Command (Centcom) announced the blockade on Sunday night, but said it will "not impede" vessels transiting the strait travelling to or from other countries.

BBC Verify has not found Iranian links for any of the vessels that have transited the strait on Monday and none can be identified as going to or from Iranian ports.

We are, however, relying on data transmitted by the vessels themselves, and none of these vessels have provided their origin as an Iranian port.

The Strait of Hormuz has become a focal point of the US-Israel war with Iran after Tehran effectively choked off one of the world's most important shipping lanes.

President Donald Trump says the US blockade is in response to Iran "knowingly failing" to reopen the strait.

He also warns the US Navy will "blow to hell" any Iranians that attack them and will take action against any ship found paying transit tolls to Iran.

The disruption to shipping since the conflict began six weeks ago has sent shock waves across the global economy, destabilising energy prices and exposing just how reliant international supply chains are on the channel that connects the Gulf with the Indian Ocean.

BBC Verify has tracked 23 vessels transiting the Strait of Hormuz since the breakdown of ceasefire talks between Iran and the US early on Sunday morning.

At least 16 of these are linked to an Iranian port, fly under the Iranian flag or have been sanctioned for links to Iran.

An average of 138 ships passed through the strait each day before the conflict started on 28 February, according to the multinational Joint Maritime Information Center.

Richard Meade, editor-in-chief of Lloyd's List says it has been a "very dangerous" time for ship owners who still face a huge amount of uncertainty.

"We know Iran is essentially still in control of the strait, and the assumption is that ship owners will still need to seek permission from the IRGC [Islamic Revolutionary Guard Corps]… and how that's going to work is still not clear."

If crossings for ships not linked to Iran do resume at a greater pace, Meade expects stranded tankers that are fully loaded with cargo will be the priority.

"You've had nearly 800 ships stuck in there for several weeks. Most of them are now loaded with cargo so the priority is going to be to get them out."

BBC Verify's analysis of the paths taken by the ships shows them taking a northern route through the strait close to Iran's coastline, within its territorial waters.

Prior to the conflict, vessels usually took a more southerly route through the middle of the waterway.

Another uncertainty is the possibility of sea mines, says Thomas Kazakos, secretary general of the International Chamber of Shipping.

"We need to make sure that we have clear confirmation that the safety of navigation for the ships and the seafarers are being agreed," he told BBC Verify.

Iran's Islamic Revolutionary Guard Corps (IRGC) Navy published what it states are the permitted routes to be followed when passing through the strait "to avoid possible collision with sea mines".

It has marked a region in the middle of the channel as a "dangerous area" to be avoided.

Centcom has previously announced that two destroyers - the USS Frank E Petersen and USS Michael Murphy - are in the area as part of a mission to clear "sea mines previously laid by Iran's Islamic Revolutionary Guards Corps".

13 April: This piece is being updated to reflect the latest number of vessels crossing the strait since the ceasefire.

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