"The EU is determined to preserve it," she said. "We expect the rest of the international community to continue to preserve it, for the sake of collective security."
"The nuclear accord belongs to the whole of the international community," Mogherini added. "To the Iranian people I say: do not let anyone dismantle this deal, one of the greatest achievements of the international community."
"I am particularly worried about tonight's announcement of further sanctions," she said, adding that the deal with Iran "is the culmination of 12 years of diplomacy."
The international agreement, which limited the Iran's nuclear program in return for sanctions relief, was negotiated under former U.S. President Barack Obama in 2015.
Trump, who promised to dismantle the deal during his 2016 electoral campaign, claims that Iran has continued to enrich uranium for military purposes.
The landmark deal, known as the Joint Comprehensive Plan of Action (JCPOA), was struck between Iran and the five permanent members of the United Nations Security Council -- China, France, Russia, Britain and the U.S. -- plus Germany and the EU, in Vienna in 2015.
European Council President Donald Tusk on Tuesday also expressed his disapproval over Trump's decision to pull out of the deal, saying the move "will meet a united European approach."
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