THOUGH they are unlikely to be fully implemented before the middle of the year, the prospect of fierce new sanctions on Iran has already ratcheted up tension between the Islamic republic and the West. On January 23rd the European Union is expected to confirm that it will embargo oil imports from Iran (see our chart), a fifth of Iran's overall sales. The EU's move follows the signing into law by Barack Obama on December 31st of measures, passed almost unanimously by Congress, to stop foreign financial institutions from transacting with Iran's central bank, the main conduit for the country's energy deals. The EU, with France and Britain in the lead, is also looking at other ways of hurting the central bank.

Iran has reacted by threatening to close the Strait of Hormuz, the Gulf's chokepoint through which a fifth of the world's oil passes, if the embargo goes ahead. It has also warned Saudi Arabia not to make good on its promise to boost production to offset Iranian oil removed from the world market. The sanctions, which were partly prompted by the most recent report, in November, of the International Atomic Energy Agency (IAEA), the UN's watchdog, are not, however, the only reason for Iran's ire. A car-bomb on January 11th killed a chemical engineer in charge of procurement at the Natanz uranium-enrichment plant. He was the fourth Iranian scientist to be killed in the past two years. His death followed several other recent unexplained explosions at factories and military sites, perhaps thanks to a covert campaign by Western intelligence agencies and Israel's Mossad to delay the nuclear programme. Just before the assassination, an Iranian court sentenced to death an Iranian-American former marine after convicting him of spying for the CIA. Adding to the jumpiness in Tehran—and in Washington—are warlike rumblings coming from Israel. Having at first praised the new sanctions, Israel's prime minister, Binyamin Netanyahu, and his deputy, Moshe Yaalon, lamented the failure to impose them immediately. Mr Yaalon provocatively accused Mr Obama of dragging his feet for fear of boosting oil prices in an election year.

The chances of Israel launching a unilateral strike on Iran this year may be rising. The head of Iran's Atomic Energy Organisation, Fereydun Davani-Abbasi, says enrichment of uranium to 20% purity, close to weapons-grade, has begun at Fordow, a supposedly impregnable facility that is buried deep in a mountain complex surrounded by anti-aircraft batteries near the holy city of Qom. Israel's defence minister, Ehud Barak, said in November there was less than a year to stop Iran's nuclear plans, because once most of the enrichment is being carried out at Fordow, Iran will be in “immune space” and the option of a military attack, at least by Israel acting alone, would be off the table. Though many in Israel's defence and security establishment doubt the merits of an attack, the final decision would be taken by Mr Netanyahu, Mr Yaalon and Mr Barak, all regarded as hawks. Mr Netanyahu may well also believe that in an election year Mr Obama would have to support Israel if it took such a step, whereas once safely re-elected (against Mr Netanyahu's preferences) he might not. If Iran is worried that Israel might strike, so too is the Obama administration. Though Mr Barak said on January 18th that a decision was still “very far off”, relations between the two allies over Iran are strained. In a recent telephone conversation with Israel's prime minister, Mr Obama is said to have warned strongly against an attack. The chairman of the Joint Chiefs of Staff, General Martin Dempsey, is about to visit Israel, ostensibly to share intelligence but also to ram home the message from his boss. America was also keen to distance itself from the murder of the Natanz engineer. Tacitly pointing a finger at Israel, a Pentagon spokesman said: “The United States played no role whatsoever in the killing of this scientist. We have been very clear that we seek to lower the temperature on tensions with Iran, and we think that things have calmed down a bit in recent days.” Apparently with that in mind, the White House called off a big joint war game with Israel planned for the spring, though 9,000 American troops have already arrived in Israel. But on January 13th, the Pentagon announced that it was stationing 15,000 additional forces in Kuwait. At least two carrier groups will remain on patrol in the region; a third, it is reported, may join.

The argument between America and Israel stems not from differences over whether Iran should be stopped from having a nuclear weapon; Leon Panetta, America's defence secretary, came close to promising to use force if necessary earlier this month. But their perspectives are different. Israeli hawks, such as Mr Barak, consider that the beginning of enrichment at Fordow marks the crossing of a red line which may quickly be followed by a “breakout” announcement of the kind North Korea made in 2006 when it tested a primitive nuclear device. American officials think that is improbable, given that Iran would invite an attack when it had no adequate means of deterrence. Rather, they believe Iran's approach is to keep moving every piece forward until it reaches the threshold of a capability to build a number of warheads and the missiles to deliver them. If so, there is still time for economic and diplomatic pressure to persuade Iran that crossing the threshold is not in its interest.

In truth, no one knows how likely that is. Iran has a proud record of resisting outside pressure and has dealt, albeit at some cost, with sanctions of one kind or another for over 30 years. It is also unclear how much greater pain the new sanctions will inflict on its already distorted economy. Iran will probably still be able to sell most of its oil to China and India at heavily discounted prices. And Mr Yaalon has a point when he suggests that the appetite in the West for an strict oil embargo may be limited. Even if Saudi Arabia is willing to pump a lot more oil and Libyan production is recovering faster than expected, inventories remain tight and Asian demand will quickly eat into whatever spare capacity is left. Société Générale, a bank, reckons that even if the embargo is phased in slowly, Brent crude could reach as much as $150 a barrel, as twitchy traders fret over the way the market adjusts. That might be enough to send America back into recession and worsen the euro-zone crisis.

So the new sanctions are likelier to be incremental rather than dramatic in their impact on Iran. But they will add to the country's sense of isolation at a time when it faces other threats, above all the Arab spring and the possible downfall of the Assad regime in Syria, its only solid regional ally. Nor is it clear that Iran itself has decided about reaching, let alone crossing, the nuclear threshold. It is trying hard to persuade the IAEA's inspectors to come to Iran later this month in an effort to try and allay some of the agency's concerns about the “possible military dimensions” of its nuclear programme. Even with Israel champing uneasily at the bit, so long as neither Iran nor America is seeking a do-or-die confrontation, the betting, just, is that there will not be one.