Aaron David Miller, a former US State Department adviser, said Trump had talked tough against Iran but was actually "risk averse" when dealing with the regime and would likely take the safer course over waiving sanctions relief.
The White House said Trump had "underscored that Iran must stop its destabilizing activity in the region".
Boris Johnson has raised the plight of imprisoned British mother Nazanin Zaghari-Ratcliffe with his Iranian counterpart on the margins of an global summit.
US President Donald Trump wants to amend the deal or withdraw from it.
Refusing to certify that the agreement is in the best interest of the US allows Congress to weigh in but has no direct effect on the deal itself.
The deal is one of several foreign policy issues where Europe's most powerful economies stand in opposition to the Trump administration.
European Union foreign ministers, including Boris Johnson, met their Iranian counterpart, Mohammad Javad Zarif, in Brussels yesterday. The sanctions relief granted under the nuclear deal opened world oil markets to Iran and allowed it to return to worldwide banking systems and trade, a key boost to its battered economy. "And we expect all parties to continue to fully implement this agreement".
He called the deal a "considerable diplomatic accomplishment" and called for worldwide support for the deal. All of the other participants in the deal - including USA allies in Europe - have said Iran is honoring its end of the agreement and that they oppose ending or reopening it. "It is incumbent on those who oppose the JCPOA to come up with that better solution because we have not seen it so far".
Tonya Harding threatens to end interview with Piers Morgan
Nancy Kerrigan has better things to do right now than see " I, Tonya " - but she does want to remind the world of one thing. After a practice session on January 6, 1994, someone tried to break USA figure skating champion Nancy Kerrigan's legs.
Trump Vetoed His Trip To London Amid Fears Of City-Stopping Protests
The latest development followed reports that Mr Trump wanted to delay a potential visit amid concerns about large-scale protests. London Mayor Sadiq Khan said Trump finally got the message that he was unwelcome in the U.K, which is why he canceled his visit.
Kim's brainwashed North Korean citizens 'would fight to the death'
Kim , on the other hand, has charged that he would "tame the mentally deranged USA dotard with fire", referring to Trump. A typical round of outrage ensued, directed not at the North Korean dictator but at the president of the United States .
One would automatically re-impose, or "snap back", suspended sanctions if Iran commits certain actions, possibly including things unrelated to its nuclear program.
The Iranian government has used sanctions relief to massively increase its support of terrorism by giving $830 million to Hezbollah, $70 million to Islamic Jihad and an estimated $50 million per month to Hamas; the Houthi rebels in Yemen received missiles from Iran that they have used to attack ships in the Red Sea and other targets in Saudi Arabia.
One U.S. official said on Wednesday that if Trump waived the key sanctions under the deal, the administration would impose new, targeted measures against Iranian businesses and people.
The Iranian said there was a "strong consensus in Brussels today" that Iran was complying with the deal and that "any move that undermines (the agreement) is unacceptable". The accord has provided "clear and tangible" benefits on limiting Iran's nuclear program, according to a recently released report by US think tanks.
Iran has said that if the United States walks away from the agreement, it is ready to give an "appropriate and heavy response". A legislative "fix" Trump requested in October has not materialized.
Yet keeping the United States in the JCPOA goes against one of Trump's major campaign promises and contradicts his repeated characterization of it as one of the worst deals in history.
He accused Iran of committing "multiple violations" and promised to work with Congress to "address the deal's many serious flaws".
Trump's choice comes at a delicate time for Iran's government, which faced protests over economic hardships and corruption that are linked to frustration among younger Iranians who hoped to see more benefits from the lifting of sanctions.