Foreign Secretary Boris Johnson has said Britain will not "walk away" from the Iran nuclear deal following the dramatic withdrawal of the US from the agreement.
European diplomats were scrambling to salvage the three-year-old accord amid fears that Donald Trump's threat to impose the "highest level" of sanctions on Tehran could trigger a new confrontation in the region.
Chancellor Angela Merkel said Germany remained committed to the agreement while French foreign minister Jean-Yves Le Drian insisted it was not "dead".
The DUP's Sammy Wilson welcomed Mr Trump's decision. The East Antrim MP blamed Iran for "prolonging the conflict in Syria and promoting war in the Yemen". He said: "Iran was an out of control rogue state which was causing havoc in the Middle East."
SDLP leader Colum Eastwood branded Mr Trump's decision to withdraw from the deal as "a reckless attempt to assert his authority with no understanding of the catastrophic consequences".
He added: "Trump's delusion should worry us all. His fixation on sticking it to Obama and his determination to follow through on outrageous policies, both foreign and domestic, should be of great concern to the international community.
"I am calling on Iran and European allies to continue to preserve the deal in spite of President Trump's whimsical decision to attempt to sabotage it."
Meanwhile, former Ulster Unionist First Minister Lord Trimble has signed up to an advertisement backing Mr Trump's move.
It stated: "We stand alongside you in ending the dangerous appeasement of Iran and taking all and any action required to stop Iran going nuclear, help its people, halt its spreading of terror and achieve peace and stability in the Middle East and among all peoples and nations." In Tehran, supreme leader Ayatollah Ali Khamenei voiced defiance, saying Mr Trump "cannot do a damn thing", while legislators in the Iranian parliament burned a paper US flag.
At Prime Minister's questions Theresa May told MPs the European powers were working to address the concern which had led Mr Trump to pull out of the Joint Comprehensive Plan of Action (JCPOA).
Mr Johnson said the Government continued to believe the agreement was "vital" to UK national security and had done its "utmost" to persuade Mr Trump not to abandon it.
He said it was up to the US to spell out the way forward and he urged the administration not to take any action which would hinder the efforts of the other parties to make it work.
"For as long as Iran abides by the agreement... then Britain will remain a party to the JCPOA. Britain has no intention of walking away," he said.
"Instead we will co-operate with the other parties to ensure that while Iran continues to restrict its nuclear programme, then its people will benefit from sanctions relief in accordance with the central bargain of the deal." In a statement, the International Atomic Energy Authority - the global watchdog responsible for monitoring the agreement - said "as of today" Tehran was continuing to honour its commitments.
Mr Johnson's comments came amid fears that the reimposition of US sanctions could hit European firms - which have led the way investing in Iran under the JCPOA - particularly hard.