Secretary of State Hillary Rodham Clinton said on Tuesday that the US would consider extending a defence umbrella over the states in the Persian Gulf region if Iran does not bow to demands to halt its nuclear programme.

Her comment, delivered at a freewheeling town-hall meeting in Bangkok, was both a warning to the Iranian regime and a glimpse of how the Obama administration might cope with a nuclear-armed Iran, should Tehran continue with what Washington says is a sustained effort to acquire nuclear weapons.

A defence umbrella would move the US closer to the explicit security guarantee that Washington gives allies in Asia, though that is a nuclear umbrella — a term Hillary did not use on Wednesday. She did talk about fortifying the military capability of Iran’s neighbours.

“We want Iran to calculate what I think is a fair assessment that if the US extends a defence umbrella over the region, if we do even more to support the military capacity of those in the Gulf,” she said. “It’s 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.”

An official said Hillary’s remarks did not reflect a shift in the administration’s policy of preventing Iran from obtaining a weapon. “She is making an argument to Iran about why they should not do this,” the official said.