(Adds comments by Trump, analyst, background)
By Steve Holland and Matt Spetalnick
WASHINGTON, Feb 1 (Reuters) - The White House put Iran "on notice" on Wednesday for test-firing a ballistic missile and said it was reviewing how to respond, taking an aggressive posture toward Tehran that could raise tensions in the region.
While the exact implications of the U.S. threat were unclear, the new administration signaled that President Donald Trump intended to do more, possibly including imposing new sanctions, to curb what he sees as defiance of a nuclear deal negotiated in 2015 by then-President Barack Obama.
The tough talk commits the administration to back up its rhetoric with action, which could cast doubt on the future of the Iran agreement and sow further uncertainty in an already chaotic Middle East, experts said.
Trump has frequently criticized the Iran nuclear deal, calling the agreement weak and ineffective.
Officials declined to say whether the military option was on the table, although Pentagon spokesman Christopher Sherwood said: "The U.S. military has not changed its posture in response to the Iranian test missile launch" on Sunday.
A fiery statement from Trump's national security adviser, Michael Flynn, marked some of the most aggressive rhetoric by the administration that took office on Jan. 20, making clear that Obama's less confrontational approach toward Iran was over.
Flynn said that instead of being thankful to the United States for the nuclear deal, "Iran is now feeling emboldened."
"As of today, we are officially putting Iran on notice," he told reporters in his first appearance in the White House press briefing room.
He said the launch and an attack on Monday against a Saudi naval vessel by Iran-allied Houthi militants off the coast of Yemen underscored "Iran's destabilizing behavior across the Middle East."
Iran confirmed it had tested a new missile but said it did not breach a nuclear accord reached with world powers or a U.N. Security Council resolution that endorsed the pact.
'HOW WOULD U.S. RESPOND?'
Analysts said Iran could interpret Flynn's warning as bluster given that the Trump administration is still formulating a response.
"It's a vague way of drawing a line in the sand," said Mark Fitzpatrick, executive director of the International Institute for Strategic Studies-Americas. "Taken literally, it could mean: 'You do this one more time and you'll pay for it.' But how would the U.S. respond?"
The warning could foreshadow more aggressive economic and diplomatic measures against Iran.
Three senior U.S. officials, speaking on condition of anonymity, said a range of options, including economic sanctions, was being considered and that a broad review was being conducted of the U.S. posture toward Iran.