WASHINGTON — U.S. Secretary of State Mike Pompeo warned Monday that the United States would impose "the strongest sanctions in history" against Iran if it did not agree to change course.

"We will apply unprecedented financial pressure on the regime," Pompeo said in his first major foreign policy address, delivered at the conservative Heritage Foundation. "The leaders in Tehran will have no doubt about our seriousness."

Pompeo outlined an alternate path: reprieve from sanctions and restoration of full diplomatic and economic relations should Iran meet a list of 12 demands aimed at the heart of Iran's foreign policy agenda.

Among other things, Iran must end all military aspects of its nuclear program, stop uranium enrichment, completely withdraw from Syria and end support for terrorist groups, Pompeo said. Pompeo also demanded the release all U.S. citizens and those of U.S. partners and allies "detained on spurious charges or missing in Iran."

"While to some, the changes in Iranian behavior we seek may seem unrealistic, we should recall that what we are pursuing was the global consensus before the JCPOA," said Pompeo, referring to the Joint Comprehensive Plan of Action, the 2015 nuclear deal with Iran adopted under the Obama administration. "We are not asking anything other than Iranian behavior that is consistent with global norms and the elimination of its capacity to threaten our world with its nuclear activities."

The Trump administration ended U.S. participation in the JCPOA — an agreement that also includes Britain, France, Germany, Russia and China as signatories — this month after concerted efforts by European partners to salvage the deal.

"I have spent a great deal of time with our allies in the past week, and they may try to keep the old nuclear deal going with Tehran. That is their decision to make," Pompeo said. "They know where we stand."

Now, the secretary says the U.S. is pursuing a bigger Iran deal with a broader international coalition, asking for the support of U.S. allies beyond Europe, such as Australia, Japan, Jordan and Saudi Arabia.

Speaking at an event in Argentina, U.K. Foreign Secretary Boris Johnson called Pompeo a "great guy" but said he thought a new "jumbo" Iran treaty would be very difficult to achieve.

"I think if you try now to fold all those issues — ballistic missiles, Iran's behavior, Iran’s disruptive activity in the region, nuclear activity — if you try to pull all of those into a giant negotiation, a new jumbo Iran negotiation, a new treaty," Johnson said, "I don’t see that being very easy to achieve in anything like a reasonable timescale."

Pompeo said the timeline for change is up to the Iranian people.

"At the end of the day the Iranian people get to make a choice about their leadership," Pompeo said in response to a question from the audience. "If they make the decision quickly, that would be wonderful. If they choose not to do so, we will stay hard at this until reaching the outcome I set forward."

The Pentagon also seemed to suggest a more aggressive policy on confronting Iran Monday, saying that they would "double down" on actions to confront Iran’s "malign influence" in the region.

"We are going to take steps necessary to address Iran’s influence in the region," Col. Rob Manning, a Pentagon spokesman, told reporters. "It is a whole of government approach."

Manning did not outline concrete steps that the Pentagon might take, however.

"It's on the table. We are not going to rule out anything," he said.