There are hints that the US may enforce its current sanctions laws against foreign companies dealing with Iran far more aggressively than it has done yet — and Congress may tighten the laws further. That could hurt Iran and companies dealing with it more than anything the UN laboriously does.

This week Britain, along with the other permanent members of the UN Security Council, will try to thrash out ways to put pressure on Iran for failing to curb its nuclear ambitions. But hopes aren’t high.

Now that the EU has dropped its insistence that Iran stop the most controversial nuclear work before talks begin, the combined US-EU negotiating position is “all over the place”, in the words of one official.

It is possible that this muddle will distil into coherent sanctions, beginning with travel bans on the political elite, to pick everyone’s favourite item off the menu of empty diplomatic gestures. But almost by definition, if the council agrees to pass them, it will be because they have no effect.

That is not true of the US’s own efforts. For a start, there is a new, tougher tone towards commercial contact with Iran. Henry Paulson, the Treasury Secretary, used the weekend meetings between finance ministers from the Group of Seven leading economies to call for help in choking off funds to Iranian companies that the US suspects of trading in weapons or nuclear components.

He said that he had been shocked by intelligence suggesting that more than 30 front companies could be involved in a suspected network and that they may have duped Western banks into helping inadvertently. “These are companies that are mundane-sounding businesses that do many legitimate activities, but in addition do many untowards and illicit activities,” he said.

For ten years the US has threatened penalties against companies who do business with Iran under its Iran-Libya Sanctions Act. “Ilsa”, as it is known, bars non-US oil and gas companies from investing more than $20 million (£11 million) a year in Iran. US companies are banned separately from any dealing with Iran by presidential directive.

The law has always been controversial among non-US companies, as it asserts the right to punish them for dealing with Iran even if their own countries have no objection — and it targets them directly, not Iran.

This summer, in recognition of Libya’s decision to give up a clandestine nuclear programme, Congress redrafted Ilsa to excise the part about Libya. Isla is due to expire on September 29, but a revised version, dealing with just Iran, is all but certain to be passed soon, fanned by the campaigns for the November congressional elections.

There are anecdotal signs that the US may enforce this version more vigorously, and interpret its provisions more widely than in the past. There has been a flurry of activity under the US’s separate Iran Non-Proliferation Act 2000, targeting companies from Russia, North Korea, India and Cuba for allegedly supplying banned equipment.

Energy companies have been relaxed about Ilsa in the past. It has deterred some from dealing with Iran, but many have flouted it, assuming that the US would not choose to pursue them. French and Japanese oil companies are among those who have looked recently at making extensive investments.

But if the US took an even tougher approach than it has in the past, they might be more wary. And now there is a chance that Congress will produce a much tougher piece of legislation altogether.

Early this year the House of Representatives passed the Iran Freedom Support Act, a much tougher sanctions package on energy companies investing in Iran. The White House, not known for indulgence towards Iran, objected on the ground that the Bill could jeopardise the diplomacy.

The Senate has not yet passed its version and so, for the moment, the Bill languishes. But these exceptionally bitter elections could revive it, as could any collapse of the diplomatic efforts. Iran’s high excitement this week at the Pope’s supposed slur on Islam will only add to the heat.

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