WASHINGTON – President Donald Trump threatened Tuesday to use "overwhelming force" against Iran if it attacks U.S. assets or personnel.

"Any attack by Iran on anything American will be met with great and overwhelming force. In some areas, overwhelming will mean obliteration," Trump wrote on Twitter.

The threat came less than a week after Trump called off a planned U.S. missile strike on Iran that the military prepared in retaliation for the shooting down of an American drone by Tehran.

His remarks marked a sharp pivot from the softer tone the president used in recent days toward Iran, in which he urged negotiations over military action and boasted that he was seen as a "dove" after canceling the strike last week.

Trump returned to the subject of Iran during an event in the Oval Office on Tuesday, reiterating his commitment that his administration would not allow Tehran to obtain nuclear weapons. Asked whether he had an exit strategy should a conflict arise with Iran, the president responded, "I don't need exit strategies."

Still, the president said he remains willing to talk with Iranian officials.

“When they’re ready, they’ll let us know,” Trump said. "Whatever they want to do, I’m ready.”

The Islamic republic's leaders mocked new U.S. sanctions and said they prevented negotiations.

“The fruitless sanctions on Iran’s leadership and the chief of Iranian diplomacy mean the permanent closure of the road of diplomacy with the frustrated U.S. administration,” said Abbas Mousavi, a Foreign Ministry spokesman, according to the state-run Islamic Republic News Agency

Iranian President Hassan Rouhani blasted the sanctions against Iran's supreme leader, Ayatollah Ali Khamenei, as “outrageous and idiotic,” noting the 80-year-old Shiite cleric has no plans to ever travel to the USA. In remarks broadcast on Iranian state television, Rouhani said the White House suffers from "mental retardation."

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Trump called those statements "very ignorant and insulting."

Trump's threat to obliterate Iran may be met with skepticism. Some foreign policy experts criticized his decision last week to pull back on a retaliatory military strike against Iran, saying the president looked like a "Twitter tiger" and the move was part of a broader pattern of bellicose rhetoric followed by minimal action, resulting in confusion.

"This is part of a pattern with Trump, who roars like a tiger but usually acts like a scaredy-cat," Max Boot, a senior fellow for national security studies at the Council on Foreign Relations, wrote Monday in a column for The Washington Post.

He noted that Trump vowed to rain "fire and fury" on North Korea, only to sit down with that country's repressive ruler, Kim Jong Un, and declare the two had fallen "in love." This year, he threatened to close the U.S. border with Mexico over illegal immigration but never did.

"Trump is a Twitter tiger whose threats cannot be taken seriously," Boot wrote.

Still, lawmakers in Congress raised alarms about the Trump administration's increasingly aggressive military posture. The Pentagon has dispatched additional troops and B-52 bombers, among other assets, to the Middle East.

Tuesday, Reps. Ro Khanna, D-Calif., and Matt Gaetz, R-Fla., introduced an amendment to the annual defense authorization bill that would bar any federal funds from being used for military force against Iran without congressional authorization. A similar proposal may come up in the Senate as early as this week.

"One of the best ways to avoid bumbling into war, a war that nobody wants, is to have a robust open debate and for Congress to have a real say," Sen. Charles Schumer, D-N.Y., said last week after a briefing on Iran at the White House.

Trump announced what he called "hard-hitting" new sanctions on Iran at the White House on Monday. He said the measures would deny Iran's leaders access to financial instruments. The penalties target Khamenei, his office and other top Iranian officials.

The measures will block transactions involving any property and other assets Iran's leaders hold in the USA.

Experts such as Ariane Tabatabai at the Rand Corp. said they would have little practical effect.

Trump unveiled the sanctions as Secretary of State Mike Pompeo traveled to the Middle East to meet with U.S. allies in the region, including Saudi Arabia and the United Arab Emirates. Tuesday, Pompeo headed to an undisclosed location, according to a State Department pool report.

Tensions between the United States and Iran have been rising for more than a year, beginning when Trump unilaterally withdrew from a multilateral deal with Iran aimed at limiting its nuclear program. Under that agreement, the United States and other countries lifted crippling sanctions on Iran in exchange for caps on its ability to enrich uranium and other restrictions.

The Trump administration reimposed those sanctions, aiming to cut Iran's oil exports to zero and force the regime to negotiate a broader agreement that would curb Iran's ballistic missile program and its support for regional terrorist groups.

As his plane is reflected in the glass building behind them, Secretary of State Mike Pompeo, right, walks with Abu Dhabi Assistant Foreign Ministry Undersecretary for Protocol Affairs Shihad Al Faheem, as they say goodbye on the secretary's departure from Abu Dhabi, United Arab Emirates, Tuesday, June 25, 2019, en route to an undisclosed location.

Iran and other parties to the agreement have tried to salvage it, but that effort has faltered, particularly as the U.S. sanctions started to crush Iran's economy. In recent weeks, the friction between the United States and Iran sharply escalated as Trump administration officials blamed Iran for a series of attacks on oil tankers in the Persian Gulf.

Iran denied sabotaging those ships, but last week, Iranian officials said they shot down a U.S. drone that crossed into its airspace. The Pentagon said the unmanned surveillance aircraft was in international airspace.

Trump said the new sanctions were in part a retaliation for the drone strike.

Trump has made it clear he wants to negotiate with Iran, but Iran has rejected his entreaties. Tehran said he is not a reliable negotiator after he withdrew from the 2015 nuclear deal.

Contributing: The Associated Press

This article originally appeared on USA TODAY: Donald Trump threatens 'overwhelming force' against Iran if it attacks 'anything American'