WASHINGTON — The White House has issued a cryptic warning that the U.S. will act against Iran unless it stops testing ballistic missiles and supporting Houthi rebels in Yemen, but declined to say what retaliatory actions the U.S. would pursue.

Michael Flynn, President Donald Trump’s national security adviser, 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 was being considered.

Later Wednesday, Trump tweeted: “Iran is rapidly taking over more and more of Iraq even after the U.S. has squandered three trillion dollars there. Obvious long ago!”