Iranian officials responded defiantly Thursday to the Trump administration’s threat to take a much tougher line over Tehran’s ballistic missile tests and support for terrorism-related activities, as the White House reportedly was preparing a slate of new sanctions against the Islamic republic as early as Friday.

While the White House declined to discuss its plans, sources familiar with the situation said the sanctions could target as many as 25 Iranian entities, signaling the most dramatic escalation in tensions between the U.S. and Iran since the signing of the 2015 accord to curb Iran’s nuclear programs in exchange for a lifting of international sanctions.

Iranian officials vowed Thursday to “forcefully” advance their nation’s ballistic missile program, claiming it was not included in the nuclear deal. A posting on the state-run FARS news service declared that it was Tehran that was putting “the U.S. on notice” — not the other way around.

The wording was deliberate. President Trump’s national security adviser, Michael Flynn, made his first appearance in the White House briefing room Wednesday to announce that he was “officially putting Iran on notice” over a “provocative” ballistic missile test and a recent attack on a Saudi naval vessel by Iran-backed rebels in Yemen.

Mr. Flynn said the Jan. 29 missile test was a clear violation of a U.N. Security Council resolution and lamented that instead of being “thankful” to Washington for the 2015 nuclear accord, Iran “is now feeling emboldened.”

Asked at a White House event about military action against Iran as the tensions escalated, Mr. Trump told reporters that “nothing is off the table.”

Earlier in the day, he posted a series of tweets slamming Iran’s behavior and the folly of the nuclear accord.

The 2015 nuclear deal eased U.S. and European sanctions on Tehran in exchange for reductions to Iran’s nuclear enrichment program — a development that Secretary of State John F. Kerry hailed as a success toward preventing the Islamic republic from developing a nuclear weapon. Major European powers, China and Russia also signed on to the deal, but Israel staunchly opposed it.

White House press secretary Sean Spicer said the Trump administration had stepped up its rhetoric to make “sure that [Iranian leaders] understood that we weren’t going to sit by and not act.”

However, neither Mr. Flynn, Mr. Spicer nor Mr. Trump offered any specifics about what being “on notice” meant for U.S.-Iranian relations. Mr. Trump has called the agreement the “worst deal ever,” talking at times during the election campaign about scrapping or demanding major changes in the agreement.

Reuters first reported Thursday evening about the plan to impose new sanctions, citing sources who said the move would not violate or conflict with the 2015 nuclear deal.

The deal included strict guidelines by which the U.S. and other nations can level sanctions against Tehran, and analysts have expressed concern that a hasty move by the Trump administration could trigger resistance from the other nations involved. International corporations, including U.S. aviation giant Boeing Co., have been flocking to Iran since the agreement was signed trying to strike a major deal with the now-open Iranian economy.

Sanction targets

The sanctions are believed to target eight Iranian entities for terrorism-related activities and another 17 for ballistic missile-related activities under separate existing U.S. executive orders. The source declined to name the entities.

The sanctions, which would block anyone with assets in U.S. markets from doing business with the designated Iranian entities, have been in the works for some time, sources said, and Iran’s decision to test-fire a ballistic missile on Sunday helped trigger the Trump administration’s decision to implement them.

Congressional Republicans, most of whom opposed the nuclear accord with Iran, have introduced several bills over the past two years for more sanctions on Tehran — replacing sanctions removed under the nuclear deal and punishing Iran for what critics say is its support of destabilizing activity in the Middle East, from Syria to Yemen.

But Iranian leaders were dismissive of the U.S. rhetoric. A top aide to Iranian Supreme Leader Ayatollah Ali Khamenei said Tehran will continue its ballistic missile tests.

“The missiles tested by Iran are for defense purposes, and the country will continue its missile activities forcefully,” senior Khamenei aide Ali Akbar Velayati told reporters in Tehran, a day after Iranian Foreign Minister Mohammad Javad Zarif warned Washington to avoid escalating tensions over the issue.

There was even a sense that Tehran was testing Mr. Trump and his team early in his presidency to see whether the White House would back up its tough talk with action.

Iran’s acting commander of the elite Revolutionary Guard Corps, Gen. Hossein Salami, told the semi-official Tasnim news agency Thursday that “if our missile power was not such a power to put fear into the hearts of Americans, there is no reason for these controversies.”

Republican lawmakers in Washington dispute Iran’s arguments that the series of missile tests do not violate the nuclear accord and are part of its legitimate self-defense efforts. House Foreign Affairs Committee Chairman Edward R. Royce, California Republican, said Wednesday that Sunday’s missile test by Iran was a clear violation of U.N. Security Council Resolution 2231, which was adopted in 2015 to make the nuclear accord official.

According to a summary on the U.N. Security Council’s website, the annex calls upon Iran “not to undertake any activity related to ballistic missiles designed to be capable of delivering nuclear weapons, including launches using such ballistic missile technology.”

Senate Foreign Relations Committee Chairman Bob Corker, Tennessee Republican, said he anticipates that Washington will engage in a “coordinated, multifaceted effort to push back” against Iran’s most recent ballistic missile test and its other destabilizing actions.

“[The Trump administration] has always thought the Iran deal, as I have, was a bad deal,” Mr. Corker said during an appearance on MSNBC. “The former administration should have taken action [and] did not take action. And I think now [the Trump administration] is even more anxious to make sure that we push back against what Iran has been doing.”

Republican lawmakers argued last year that two medium-range ballistic missile tests, which Tehran carried out in October and November of that year, had violated a 2010 U.N. Security Council Resolution that was in effect up until January 2016, when the nuclear accord became official.

But supporters of the nuclear deal said the Trump White House was making the situation worse by making vague, open-ended threats with little international support.

“Michael Flynn’s ‘notice’ to Iran [on Wednesday] clearly shows he knows not how to handle Tehran,” Trita Parsi, who heads the National Iranian American Council, said in a statement Thursday. “He would be wise to review how John Kerry managed to successfully deal with Iran without unnecessary bluster.”