The decision to impose an EU oil embargo on Iran, agreed on Monday by European foreign ministers, sets a potential bomb ticking, timed to detonate on 1 July.

On that day, according to the package of measures on the table in Brussels, Europe will stop importing oil from Iran, about a fifth of the country's total exports. At about the same time, US sanctions targeted at the global financing of Iran's oil trade will also kick in. Iran could still export some of its oil to Asia, but at big discounts.

Unlike previous sanctions on Iran, the oil embargo would hit almost all citizens and represent a threat to the regime. Tehran has long said such actions would represent a declaration of war, and there are legal experts in the west who agree.

The threat of an immediate clash in the Gulf appeared to recede over the weekend when the USS Lincoln aircraft carrier and its task force, including the British frigate HMS Argyll and a French warship, travelled through the strait of Hormuz without incident. This was despite warnings earlier this month from the Iranian Islamic Revolutionary Guard Corps (IRGC) that it would oppose the return of a US carrier to the region.

But tensions are almost certain to build again as the effective date of the oil sanctions approaches. The US has already begun beefing up its military presence in the region, and the IRGC is planning new naval war games next month. Rear Admiral Ali Fadavi told the Fars news agency earlier this month that the upcoming exercises, codenamed "the Great Messenger", would be different from previous war games, without going into detail.

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