The United States has not instituted a policy of regime change or collapse in Iran, U.S. Defense Secretary Jim Mattis said on Friday, telling reporters that the goal was still to change Iran's behavior in the Middle East.

Asked whether the Trump administration had created a regime change or collapse policy, Mattis said: "There's none that's been instituted."

"We need them to change their behavior on a number of threats that they can pose with their military, with their secret services, with their surrogates and with their proxies," he added during an off-camera briefing at the Pentagon.

Mattis' remarks were the most detailed ones by a senior official in President Donald Trump's administration about U.S. Iran policy following high-level discussions at the White House on Thursday that included the issue of Iran.

They came after a week of heated rhetoric between Washington and Tehran that was kicked off by a warning from Iran's President Hassan Rouhani on Sunday that hostile U.S. policies could lead to "the mother of all wars."

On Sunday night, Trump said in a Twitter posting written in all capital letters directed at Rouhani: "Never, ever threaten the United States again or you will suffer consequences the likes of which few throughout history have ever suffered before. We are no longer a country that will stand for your demented words of violence & death. Be cautious!"