Hillary Clinton today signalled a significant shift in US foreign policy by discussing publicly how a nuclear-armed Iran could be contained in the Middle East.

Until today, the shared position of the US, Britain and France was that Iran would not be allowed to acquire nuclear weapons, and no senior official from any of the three countries would discuss the option of containment.

However, Clinton broke that taboo during a visit to Thailand, when she pledged enhanced US protection for Washington's Gulf allies, implying nuclear protection, if Iran succeeded in building a bomb.

"We... have made it clear that we'll take actions, as I've said time and time again, crippling action working to upgrade the defences of our partners in the region," Clinton told Thai television.

"We want Iran to calculate what I think is a fair assessment: that if the United States extends a defence umbrella over the region, if we do even more to develop the military capacity of those (allies) in the Gulf, it is unlikely that Iran will be any stronger or safer because they won't be able to intimidate and dominate as they apparently believe they can once they have a nuclear weapon."

Sir Richard Dalton, a former British ambassador to Tehran said: "She's implying that, if Iran became a nuclear weapon state, then the US would develop their existing defence commitments and that the US would contemplate nuclear deterrence to protect Persian Gulf states."

The remarks appeared to be aimed at reassuring Arab allies uneasy about the rise of a nuclear Iran, and considering their own nuclear options. It also seeks to influence the decisions being made in Tehran. But it drew an immediate riposte from Israel's minister of intelligence and atomic energy, Dan Meridor.

"I was not thrilled to hear the American statement...that they will protect their allies with a nuclear umbrella, as if they have already come to terms with a nuclear Iran. I think that's a mistake," Meridor said.

Clinton's comments appear to reflect a new US pessimism on Iran following the June presidential elections, that brought an entrenchment by hardliners in Tehran.

There had been hope in Washington that Barack Obama's warm overtures to the Iranian people and the offer of talks with Iran without preconditions would break the long-running impasse over Iranian enrichment of uranium.

Iran insists the uranium is intended for peaceful power generation, but the UN security council has called for it to be suspended, at least until doubts over Tehran's intentions are resolved.

Iran had put off, until after the recent election, a response to the latest offer from six major powers – the US, Britain, France Germany, Russia and China – to offer economic help and technical assistance in building a nuclear power industry, if Iran suspends enrichment. There is little hope left in Washington or other western capitals that any response now will be positive.

The outgoing director of the International Atomic Energy Agency, Mohamed ElBaradei, said recently he thought Iran was developing a breakout capacity to build nuclear weapons, so that it would have all the components in place to build a warhead at short notice.

The containment option is boosted by the belief that Iran is running out of uranium ore to convert and enrich. The US thinks the supply will run out by next year and is urging all uranium-producing countries to tighten control over their exports, to ensure Iran does not get hold of any more.

If that effort is successful, it would severely limit the size of arsenals Iran is able to build. That is the theory at least. It is very much "plan b" as far as the west is concerned, but it is a bow to new realities.