WASHINGTON — As the Trump administration readies to re-impose sanctions on Iran that were lifted under the 2015 nuclear accord, America’s European allies fear greater regional instability.

President Donald Trump’s withdrawal from the landmark agreement, signed by the U.S. and five other world powers, remains one of the most consequential foreign policy decisions of his presidency.

Secretary of State Mike Pompeo said Sunday that renewed U.S. sanctions on Iran will be rigorously enforced and remain in place until the Iranian government radically changes course.

While calling sanctions an important pillar in U.S. policy toward Iran, he said the administration is open to looking beyond sanctions but that would “require enormous change” from Tehran.

“We’re hopeful that we can find a way to move forward but it’s going to require enormous change on the part of the Iranian regime,” Pompeo told reporters aboard his plane on his way home from a three-nation trip to South East Asia. “They’ve got to behave like a normal country. That’s the ask. It’s pretty simple.”

Pompeo called the Iranian leadership “bad actors” and said Trump is intent on getting them to “behave like a normal country.”

Many U.S. allies believe that such language is code for regime change, according to two European diplomats involved in negotiations with the Trump administration over how sanctions would be re-imposed.

The sanctions that go back into effect on Monday cover Iranian trade in automobiles and metals, including gold. The U.S. also has banned imports of Iranian products such as carpets and pistachios and revoked licenses that allowed Iran to purchase U.S. and European aircraft.

Iran acquired five new European commercial planes on Sunday before the sales were cut off.

The last and most significant sanctions — those on Iran’s oil sector and central bank — will be restored on Nov. 4. Iranian oil sales are a crucial source of hard currency.

The nuclear deal lifted international sanctions in exchange for Iran agreeing to restrictions on its nuclear program. U.N. inspectors said Iran was complying with the deal, but Trump argued that it didn’t do enough to curb Iran’s malign activity in the region. Trump administration officials also argued that because the U.S. lifted sanctions against Iran as part of the agreement, it in effect stripped Washington of one of its most powerful tools to penalize Tehran.

European countries say they remain committed to the agreement, seeing it as the surest way to safeguard their national security.

“The problem is: What next?” one of the European diplomats said, referring to concerns that the U.S. is eyeing regime change as the sanctions’ end goal. Both diplomats spoke on condition of anonymity, as they were not authorized to brief the media on ongoing negotiations.

If the re-imposed sanctions caused the government in Tehran to collapse, Iran would likely devolve into civil war like what unfolded in Syria or radicals would assume power, the diplomat said.

A deepening of Iran’s economic crisis could also lead to an influx of refugees and migrants into Europe like that seen on the heels of the Syrian conflict.

Pompeo noted that the U.S. has long designated Iran as the world’s foremost state sponsor of terrorism and that Iran cannot expect to be treated as an equal in the international community until it halts such activities.

“Perhaps that will be the path the Iranians choose to go down,” he said Sunday. “But there’s no evidence today of a change in their behavior.”

In the meantime, he said, “we’re going to enforce the sanctions.”

Supporters of the Iran agreement have long argued that the U.S. departure would alienate European allies who partnered with the U.S. in the negotiations.

We “remain firmly committed to ensuring (the deal) is upheld and we continue to abide by our commitments,” the second European diplomat said. “If we cannot fulfill these, this risks Iran deciding that it no longer has to abide by the restrictions.”

Iran’s economy was plunged into a downward spiral following Trump’s announcement that the United States was scrapping the nuclear deal. The downturn has sparked waves of protests across Iran.