By Arshad Mohammed, Matt Spetalnick and Patricia Zengerle

WASHINGTON (Reuters) - U.S. President Donald Trump is poised to impose new sanctions on multiple Iranian entities, seeking to ratchet up pressure on Tehran while crafting a broader strategy to counter what he sees as its destabilizing behavior, people familiar with the matter said on Thursday.

In the first tangible action against Iran since Trump took office on Jan. 20, the administration, on the same day he insisted that “nothing is off the table,” prepared to roll out new measures against more than two dozen Iranian targets, the sources said. The announcement is expected as early as Friday, they said.

The new sanctions, which are being taken under existing executive orders covering terrorism and weapons of mass destruction, may mark the leading edge of a more aggressive policy against Iran that Trump promised during the 2016 presidential campaign, the sources, who had knowledge of the administration's plans, said.

But the package, targeting both entities and individuals, was formulated in a way that would not violate the 2015 Iran nuclear deal negotiated by Trump’s predecessor Barack Obama, they added.

The sources said the new sanctions had been in the works for some time and that Iran's decision to test-fire a ballistic missile on Sunday had helped trigger Trump's decision to impose them, although Washington has not accused Iran of violating the nuclear deal.

The White House signaled a tougher stance toward Iran on Wednesday when Michael Flynn, Trump's national security adviser, said he was putting Iran "on notice" after the missile test and senior U.S. officials said the administration was reviewing how to respond.

A top adviser to Iranian Supreme Leader Ayatollah Ali Khamenei said his country would not yield to "useless" U.S. threats from "an inexperienced person" over its ballistic missile program. The adviser, Ali Akbar Velayati did not identify a specific U.S. official in his comments.

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The impact of the new round of sanctions will be more symbolic than practical, especially as the move does not affect the lifting of broader U.S. and international sanctions that took place under the nuclear deal. Also, few of the Iranian entities being targeted are likely to have U.S. assets that can be frozen, and U.S. companies, with few exceptions, are barred from doing business with Iran.

But the administration is working with congressional staffers and outside experts on a still-evolving comprehensive plan aimed at hitting as many of Iran’s pressure points as possible, including its already restricted nuclear program, its missile development and its support of militant groups in the region.

Though development of the new approach is in its early stages, options under consideration include exercising “zero tolerance” for even the most minor Iranian violations of the nuclear deal and officially designating Iran’s Revolutionary Guard Corps as a terrorist entity, the sources said.

“Michael Flynn did not put Iran on notice as mere empty words,” said Mark Dubowitz, an Iran sanctions expert and head of the conservative Washington-based Foundation for Defense of Democracies who is advising the Trump administration and lawmakers.

"Iran's continued missile and terrorism activities will lead to dozens of new U.S. designations and tough new congressional sanctions. This is merely the beginning of what Flynn meant.”

"NOTHING IS OFF THE TABLE"

Trump’s declaration that nothing has been ruled out in response to Iran appears to leave open the possibility of military action, though experts say both sides will take care to avoid armed confrontation in the oil-rich Gulf. Still, the U.S. threats of reprisals, coupled with Iran’s defiant reaction, could dangerously ratchet up tensions between the two countries.

Every recent U.S. president, including Democratic predecessor Obama, has said that U.S. military options were not off the table to prevent Iran from getting a nuclear weapon. But Trump and some of his aides have gone much further in their rhetoric, especially in criticizing the Iran deal as weak, ineffective and in need of renegotiation.

Iran’s continued missile testing has been a source of controversy.

In the latest move, one source, who spoke on condition of anonymity, said about eight Iranian entities were to be sanctioned, or "designated" in U.S. legal jargon, for terrorism-related activities and about 17 for ballistic missile-related activities under separate existing U.S. executive orders. The source declined to name the entities, which were targeted under executive orders signed by George W. Bush in 2001 and 2005.

Sanctions designations can lead to asset freezes, travel bans and other penalties.

The White House declined comment.

A U.S. State Department official, asked for comment, replied: “As standard policy, we do not preview sanction decisions before they are announced.”

Leading a chorus of Republican calls for new sanctions, Paul Ryan, the speaker of the House of Representatives, said the United States should stop "appeasing" Tehran.

"I would be in favor of additional sanctions on Iran," Ryan told reporters at a weekly news conference. "I’d like to put as much toothpaste back in the tube as possible. I think the last administration appeased Iran far too much."

Like every Republican in Congress, Ryan opposed the nuclear agreement with Iran that went into effect early last year. But Republican lawmakers said they were working with the Trump administration to push back on Iran as much as possible without risking the international uncertainty that would come with tearing up the pact.

"Now we have a partner that's willing to deal with Iran in the way that Iran should be dealt with. ... And so we're in a real different ball game," Senator Bob Corker, chairman of the Senate Foreign Relations Committee, told Reuters.

Corker said his committee was "in the early stages" of working on legislation related to the nuclear issue.

Trump's administration has already begun looking at actions it could take without waiting for Congress, where Republicans would have to win some Democratic support to pass any new sanctions package, congressional aides said.

For example, Trump could impose sanctions already authorized by existing laws, but which were not put into effect by the Obama administration.

(Additional reporting by Jonathan Landay; Writing by Matt Spetalnick and Patricia Zengerle; Editing by Yara Bayoumy and James Dalgleish)