President Trump, speaking to reporters Sunday, said, “Iran better be careful.”
Secretary of State Mike Pompeo warned that Iran’s decision would “lead to further isolation and sanctions.”
Pompeo said on Twitter that a nuclear-armed Iran “would pose an even greater danger to the world.”
Iran’s leaders have repeatedly insisted that they do not seek nuclear weapons.
But the move Sunday was their latest attempt to persuade European leaders to reset the terms of the deal, which promised Tehran economic benefits in exchange for curbs on its nuclear energy program. The U.S. decision to reimpose sanctions on Iran has left the future of the accord in doubt.
Britain, France and Germany — all signatories to the deal — called on Iran to reverse its decision to ramp up uranium enrichment and to refrain from further steps to undermine the accord.
Britain accused Iran of violating the terms of the agreement. A spokesman for the British Foreign Office said Iran “must immediately stop and reverse all activities inconsistent with its obligations.”
Germany said it was waiting for more information from the International Atomic Energy Agency, whose inspectors monitor Iran’s nuclear activities.
The agency said the inspectors would report to its headquarters in Vienna “as soon as they verify the announced development.”
The announcement comes just one week after Iran breached the 300-kilogram limit on its stockpile of enriched uranium. It says it will scale back its obligations under the accord at 60-day intervals, until Europe complies with its side of the deal.
“All such steps are reversible” if European nations comply, Iranian Foreign Minister Mohammad Javad Zarif said on Twitter.
He called Sunday’s decision “remedial” and within the bounds of the accord. The agreement, also known as the JCPOA, allows for parties to abandon their commitments if they believe any or all of the other signatories are failing to uphold their end of the deal.
A senior official suggested Saturday that Iran would increase its enrichment rate to 5 percent, enough to produce fuel for the Bushehr nuclear reactor and other industries.
Analysts said the higher enrichment rate would not pose a near-term risk of proliferation but would decrease the breakout time Iran would need to manufacture enough weapons-grade uranium to assemble a nuclear bomb.
“Tehran has not started a crash program to build a bomb, but the accumulation of 5% enriched uranium will erode its breakout time,” Henry Rome, a Middle East analyst at Eurasia Group, wrote in a briefing note Sunday.
“Tehran is trying to strike a delicate balance: create a nuclear crisis sufficiently serious to force Europe to deliver economic concessions without going overboard and triggering a sanctions response from Europe or a military response from the U.S.,” Rome said. “While Iran is skilled at manipulating its nuclear program for maximum political effect, the strategy is inherently risky and volatile.”
Toluse Olorunnipa contributed to this report from Morristown, N.J.