WASHINGTON — President Donald Trump withdrew the U.S. from the landmark nuclear accord with Iran Tuesday, declaring he was making the world safer in restoring harsh sanctions. But he also dealt a profound blow to allies, deepened his isolation on the world stage and revived doubts about American credibility in the most consequential foreign policy action of his presidency.

The leaders of Germany, France and Britain, co-signers of the agreement, expressed regret and said they would try to salvage the accord with Iran. Iranian President Hassan Rouhani said he was sending his foreign minister to work with those remaining countries but warned there was only a short time to negotiate with them and his country could soon “start enriching uranium more than before.”

The 2015 accord, which lifted major economic sanctions against Iran, was specifically aimed at preventing that result. But Trump said, “The Iran deal is defective at its core.”

“If we do nothing, we know exactly what will happen. In just a short period of time, the world’s leading state sponsor of terror will be on the cusp of acquiring the world’s most dangerous weapons,” Trump said in a televised address from the White House.

He said the United States “will be instituting the highest level of economic sanction.”

Trump’s decision means Iran’s government must now decide whether to follow the U.S. and withdraw or try to salvage what’s left of the deal.

The leaders of Britain, Germany and France immediately urged the U.S. not to take any actions that could prevent them and Iran from continuing to implement the agreement.

The statement from Prime Minister Theresa May, Chancellor Angela Merkel and President Emmanuel Macron also urged Iran to “show restraint” and continue fulfilling its own obligations such as cooperating with inspections.