WASHINGTON — President Donald Trump said Thursday his administration has put Iran “on notice,” echoing comments from his top national security adviser that the U.S. will act against Iran unless it stops testing ballistic missiles and supporting Houthi rebels in Yemen.

Trump and his national security adviser, Michael Flynn, didn’t elaborate on what retaliatory actions the U.S. could pursue.

Trump tweeted, “Iran has been formally PUT ON NOTICE for firing a ballistic missile. Should have been thankful for the terrible deal the U.S. made with them!”

He added in another tweet: “Iran was on its last legs and ready to collapse until the U.S. came along and gave it a life-line in the form of the Iran Deal: $150 billion.”

As part of the nuclear deal struck during the Obama administration, Iran received access to an estimated $100 billion of its own money that had been frozen in foreign bank accounts. The U.S. did not give Iran $150 billion.

Flynn on Wednesday forcefully denounced Iran’s behavior in his first public remarks since Trump took office. He accused Iran of threatening U.S. allies and spreading instability throughout the Middle East while faulting the Obama administration for doing too little to stop the Islamic Republic.

“As of today, we are officially putting Iran on notice,” Flynn said from the White House podium.

On notice for what, Flynn didn’t say. Senior Trump administration officials said they were actively considering a “range of options” including economic measures and increased support for Iran’s regional adversaries. The officials, who briefed reporters on condition of anonymity, declined repeatedly to say whether military action is being considered.

The warning was an early manifestation of Trump’s promise of a tougher American approach to Iran. Yet administration officials emphasized that their allegations were unrelated to Iran’s obligations under the Iran nuclear deal that President Barack Obama and world leaders negotiated. Though Flynn noted Trump has criticized that deal, officials declined to say whether Trump planned to follow through on his campaign pledge to renegotiate it.

“The Obama administration failed to respond adequately to Tehran’s malign actions — including weapons transfers, support for terrorism and other violations of international norms,” Flynn said.

The White House also faulted Iran for backing Houthi rebels in Yemen who on Tuesday claimed a successful missile strike against a warship belonging to a Saudi-led coalition fighting to reinstall Yemen’s internationally recognized government. The media arm of the Shiite rebels said the vessel was believed to belong to the Saudi Arabian navy.