Four vessels with links to Iran have crossed the Strait of Hormuz despite the US naval blockade, according to ship-tracking data.

US Central Command announced that a blockade of "all maritime traffic entering and exiting Iranian ports" would come into force on Monday, but said it will "not impede" vessels passing through the strait travelling to or from other countries.

Two ships that have transited the waterway appear to have visited Iranian ports, according to data from ship-tracking site MarineTraffic.

Bulk carrier Christianna crossed the strait from the west on Monday after the blockade came into effect having called at Bandar Iman Khomeini in Iran, according to MarineTraffic.

A tanker, Elpis, which passed the strait heading east late on Monday, appears to have come from the Iranian port of Bushehr, MarineTraffic data shows. It is under US sanctions for Iran-related trade and its destination is unknown.

Our understanding of which ships are going through the strait is complicated by the fact some may have been "spoofing" their location signals or submitting incomplete or no data.

Two other vessels - tankers Rich Starry and Murlikishan that have been sanctioned by the US for links to Iran - also passed through the strait overnight.

Tracking data suggests at least three further vessels with no obvious links to Iran have crossed since the start of the blockade on Monday.

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 has said the US blockade is in response to Iran "knowingly failing" to reopen the strait.

He also warned the US Navy will "blow to hell" any Iranians that attack them and will take action against any ship found to be 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.

Jakob Larsen from the Baltic and International Maritime Council, which represents shipowners, told BBC Verify that he is concerned about the "risk of further escalation to involve direct attacks on ships" with the introduction of the US blockade.

BBC Verify tracked 18 vessels transiting the Strait of Hormuz between the breakdown of ceasefire talks early on Saturday morning and Trump's announcement of a blockade on Sunday evening.

At least 16 of these appeared to have links to Iran. Some were sailing under the Iranian flag and some appeared to have also called at Iranian ports

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

If crossings for ships not linked to Iran do resume at a greater pace, experts say 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," said Richard Meade, editor-in-chief of Lloyd's List

BBC Verify's analysis shows ships that have passed the strait have taken a northerly route close to Iran's coastline and 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".

Additional reporting by Kayleen Devlin

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

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