The US military is making preparations to begin boarding Iranian-linked vessels as US President Donald Trump doubles down on his blockade of Iranian ports. The US is preparing to seize oil tankers in the coming days, according to officials cited by the Wall Street Journal, including vessels operating outside the Gulf that are transporting Iranian oil or weapons. Hundreds of sanctioned vessels linked to Iran are currently operating around the world, potentially expanding the scope of US Navy action beyond the region.

Iranian oil tankers export most of the country’s 1.6 million barrels per day, primarily to China. During its pressure campaign against Venezuela, US forces interdicted ships connected to the country in both the Atlantic and Indian Oceans. The current list of US-sanctioned ships has been joined by further ones controlled by Mohammad Hossein Shamkhani, an Iranian oil shipping magnate.

The US has turned 23 vessels around that were attempting to pass through the Strait of Hormuz from Iranian ports, according to US Central Command, which runs military operations in the Middle East. US General Dan Caine, chairman of the Joint Chiefs of Staff, said earlier on Thursday that the US would pursue Iranian or Iran-linked vessels. The report comes after Trump publicly doubled down on the US blockade of Iranian ports until an agreement was reached, despite a brief opening of the Strait of Hormuz announced by Iran's foreign minister.

He also said that Iran "can't blackmail us" after the strait's subsequent closure following its brief opening. Iran itself has warned against any US action in the strait, with the spokesperson for Iran's central military command describing US actions as piracy. Iran's speaker of parliament, Mohammad Baqer Ghalibaf, warned in an interview on state TV that Iran was ready to respond to any US action, and labelled the US blockade "a clumsy decision born of ignorance".

The threats come amid speculation about a new round of negotiations between the US and Iran following a failed first round in Islamabad last week, with Iran's Supreme National Security Council saying it had received and is studying "new proposals" from the US. Trump has characterised conversations with Iran as "very good" despite the tensions and the gulf between the two. A ceasefire between the US and Iran is set to end on Wednesday, potentially unravelling ongoing negotiations, while the continued closure of the Strait of Hormuz is fuelling an energy crunch that international financial bodies say is putting a major strain on the global economy.