WASHINGTON (Reuters) - U.S. President Donald Trump struck a blow against the 2015 Iran nuclear agreement on Friday in defiance of other world powers, choosing not to certify that Tehran is complying with the deal and warning he might ultimately terminate it.

Trump announced the major shift in U.S. policy in a speech in which he detailed a more aggressive approach to Iran over its nuclear and ballistic missile programs and its support for extremist groups in the Middle East.

He accused Iran of “not living up to the spirit” of the nuclear agreement and said his goal is to ensure Tehran never obtains a nuclear weapon, in effect throwing the fate of the deal to Congress.

He singled out Iran’s Islamic Revolutionary Guard Corps for sanctions and delivered a blistering critique of Tehran, which he accused of destabilizing actions in Syria, Yemen and Iraq.

“We will not continue down a path whose predictable conclusion is more violence, more terror and the very real threat of Iran’s nuclear breakout,” Trump said.

Trump’s hardline remarks drew praise from Israel, Iran’s arch-foe, but was criticized by European allies.

The move by Trump was part of his “America First” approach to international agreements which has led him to withdraw the United States from the Paris climate accord and the Trans-Pacific Partnership trade talks and renegotiate the North American Free Trade Agreement with Canada and Mexico.

His Iran strategy angered Tehran and put Washington at odds with other signatories of the accord - Britain, France, Germany, Russia, China and the European Union - some of which have benefited economically from renewed trade with Iran.

Responding to Trump, Iranian President Hassan Rouhani said on Friday on television that Tehran was committed to the deal and accused Trump of making baseless accusations.

“The Iranian nation has not and will never bow to any foreign pressure,” he said. “Iran and the deal are stronger than ever.”

European allies have warned of a split with the United States over the nuclear agreement and say that putting it in limbo as Trump has done undermines U.S. credibility abroad, especially as international inspectors say Iran is in compliance with the accord.

The chief of the U.N. atomic watchdog reiterated that Iran was under the world’s “most robust nuclear verification regime.”

“The nuclear-related commitments undertaken by Iran under the JCPOA are being implemented,” Yukiya Amano, director general of the International Atomic Energy Agency said, referring to the deal by its formal name.

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U.S. Democrats expressed skepticism at Trump’s decision. Senator Ben Cardin said: “At a moment when the United States and its allies face a nuclear crisis with North Korea, the president has manufactured a new crisis that will isolate us from our allies and partners.”

CONGRESS DECIDES

While Trump did not pull the United States out of the agreement, he gave the U.S. Congress 60 days to decide whether to reimpose economic sanctions on Tehran that were lifted under the pact.

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If Congress reimposes the sanctions, the United States would in effect be in violation of the terms of the nuclear deal and it would likely fall apart. If lawmakers do nothing, the deal remains in place.

Senate Foreign Relations Committee Chairman Bob Corker was working on amending the Iran Nuclear Agreement Review Act law to include “trigger points” that if crossed by Iran would automatically reimpose U.S. sanctions.

The trigger points would address strengthening nuclear inspections, Iran’s ballistic missile program and eliminate the deal’s “sunset clauses” under which some of the restrictions on Iran’s nuclear program expire over time.

Trump directed U.S. intelligence agencies to probe whether Iran might be working with North Korea on its weapons programs.

The president, who took office in January, had reluctantly certified the agreement twice before but has repeatedly blasted it as “the worst deal ever.” It was negotiated under his predecessor, former President Barack Obama.

Trump warned that if “we are not able to reach a solution working with Congress and our allies, then the agreement will be terminated.”

“We’ll see what happens over the next short period of time and I can do that instantaneously,” he told reporters when asked why he did not choose to scrap the deal now.

The Trump administration designated the entire Islamic Revolutionary Guard Corps under an executive order targeting terrorists. The administration stopped short of labeling the group a Foreign Terrorist Organization, a list maintained by the State Department.

The Revolutionary Guard is the single most dominant player in Iran’s security, political, and economic systems and wields enormous influence in Iran’s domestic and foreign policies.

It had already previously been sanctioned by the United States under other authorities, and the immediate impact of Friday’s measure is likely to be symbolic.

The U.S. military said on Friday it was identifying new areas where it could work with allies to put pressure on Iran in support of Trump’s new strategy and was reviewing the positioning of U.S. forces.

But U.S. Defense Secretary Jim Mattis said no changes in force posture had been made yet, and Iran had not responded to Trump’s announcement with any provocative acts so far.