U.S President Donald Trump slapped fresh sanctions on Iran's weapons procurement network Friday, provoking an angry response from Tehran in what is an increasingly tense stand-off.
Officials said the new measures were in response to Iran's recent ballistic missile test and its support for the Huthi rebels in Yemen, who recently targeted a Saudi warship.
The new sanctions do not yet mean that the U.S has abandoned commitments it made under the deal to lift measures aimed at Iran's nuclear program, officials said.
But Trump has made no secret of his contempt for that accord, which his predecessor Barack Obama approved in July 2015, and officials said Friday's measures would not be the last.
"Iran is playing with fire - they don't appreciate how 'kind' President Obama was to them. Not me!" Trump tweeted.
Underlining what he sees as the tougher stance under Trump rather than Obama, hardline U.S National Security Adviser Michael Flynn drew a stark line in the sand.
"The days of turning a blind eye to Iran's hostile and belligerent actions toward the United States and the world community are over," he said.
"This behavior seems continuous despite the very favorable deal given to Iran by the Obama Administration. These sanctions target these behaviors."
Iran reacted angrily to the sanctions, vowing to impose "legal limitations" on Americans that Tehran alleges are involved in creating and supporting "extreme terrorist groups."
US intelligence and Treasury officials are constantly scrutinizing Iran's networks, looking for evidence of extremist funding and advanced weapons procurement.
The immediate trigger for the sanctions was Iran's test, on Sunday, of a ballistic missile that U.S officials judge to have been capable of one day carrying a nuclear warhead.
But the latest detailed list of designations has clearly been in preparation for some time, and the White House says "nothing is off the table" -- even military action.
"We are undertaking a larger strategic review," a senior U.S administration official told reporters. "The launch of the missile was the triggering event."
Washington is also concerned about attacks by Huthi rebels, a powerful faction in Yemen's civil war which U.S intelligence believes is armed and supported by Iran.
This week Huthi forces attacked a Saudi warship operating off Yemen and on Friday, a US official said: "We're very concerned about freedom of navigation in the Bab el-Mandeb area."
The senior official said Iran was "not necessarily responsible for every tactical decision" made by Huthi forces, but that it will be made to bear responsibility for its "proxies."