She said crossing the nuclear threshold would not make Iran, which Washington believes is pursuing nuclear weapons, safer or more secure.
"We will still hold the door open (for talks with Iran) but we also have made it clear that we'll take actions, as I've said time and time again, crippling action, working to upgrade the defense of our partners in the region," she said in a program taped for Thai television during a visit to Bangkok.
"We want Iran to calculate what I think is a fair assessment … that if the U.S. extends a defense umbrella over the region, if we do even more to support the military capacity of those in the Gulf, 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."
Last week Clinton said Iran's intentions were unclear following June's election there and that Washington's offer of talks with Tehran over its nuclear program was not open-ended.
The former Bush administration refused to engage Iran directly until it had met certain preconditions, including suspending uranium enrichment, a process that can produce fuel for nuclear power plants or atomic weapons.
But President Barack Obama, who took over in January, says that approach failed and Clinton has also said it was a mistake.
Despite the policy shift, Iran has not responded to Obama's overtures and those from other countries seeking to persuade Tehran to give up sensitive nuclear work the West believes is aimed at building a bomb and Iran says is to generate power.
Diplomats suspect Iran is buying time by stalling over getting into any substantive talks.
Clinton is heading to the Thai resort of Phuket to attend Asia's biggest annual security meeting. A key topic will be how to rein in North Korea's nuclear program.
(Reporting by Arshad Mohammed; Editing by Alex Richardson)
