EU foreign policy chief Federica Mogherini warned Tuesday that the Iran nuclear deal can never be renegotiated, as she urged US lawmakers not to try to reopen the agreement.
Her visit coincided with that of the head of the UN nuclear watchdog, IAEA director general Yukiya Amano, who defended his agency's ability to monitor Iran's compliance with the accord.
President Donald Trump has told Congress that he can no longer "certify" that the 2015 accord is in the US national interest, leaving its fate in the hands of skeptical senators.
Mogherini, therefore, came to Washington -- while Trump and her opposite number Secretary of State Rex Tillerson were touring Asia -- to sit down with legislators on Capitol Hill.
While she insisted Brussels does not want to interfere in Washington politics, she sent a clear message that Europe opposes any new US law that would breach the terms of the deal.
Senator Bob Corker, the head of the powerful Senate foreign relations committee, and his Republican colleague and rising star Senator Tom Cotton, are working on such a bill.
While it has not been finalized -- and its supporters insist it is designed to strengthen enforcement of the Iran deal rather than to destroy it -- opponents fear it will do just that.
Speaking to reporters after talks with US lawmakers, Mogherini said she had sought and received reassurances that the bill would keep the United States "compliant with the deal."
"We are exchanging views with legislators on the need to make sure, before a bill is presented, that its contents does not represent a violation," she said.
In a fact sheet to explain the proposed law, the senators propose that US nuclear-related sanctions on Iran would snap back if Tehran violates "enhanced and existing restrictions."
These "enhancements" to restrictions would end so called "sunset" clauses that would see some curbs on Tehran's program phased out between eight and 15 years from now.
But supporters of the deal argue changing these provisions of the Joint Comprehensive Plan of Action (JCPOA) signed by Iran and six world powers would itself collapse the deal.
Mogherini pushed back strongly on the idea that the 2015 deal, under which Iran submitted to nuclear controls in return for sanctions relief, would "sunset" at all.
"The JCPOA has no sunset clause," she said. "There are different arrangements within the JCPOA that have different durations... some will last forever.
"Article Three of the agreement says that Iran will never develop a nuclear weapon, for instance."
And, she added, under the "foreseen ratification" by Iran of the International Atomic Energy Agency's "additional protocol ... certain commitments will become permanent."
"Renegotiating is not an option," she said, insisting Britain, France, Germany, China and Russia will not review 12 years of complex international diplomacy because of US concerns.
Separately, IAEA chief Amano denied claims by opponents of the Iran deal that his agency is not able to monitor Iran's compliance with JCPOA and is blocked from military sites.
"I cannot discuss the details of our access, but what I can say is that we have had access to all the locations that we needed to visit and this should continue," he said.
He said the issue of access had been "exaggerated" and whether or not a site was military or civilian run was not important.
"It is not that relevant," he said. "The places that interest us are the places that have nuclear material, nuclear expertise and nuclear-related equipment."
Amano visited Iran last week and met with President Hassan Rouhani, vice president and atomic energy chief Ali Akbar Salehi and Foreign Minister Javad Zarif.
All had assured him, he said, that Iran would not be the first to pull out of the JCPOA.
This would include Iran's eventual ratification of the IAEA "additional protocol", but Amano said that inspections were already taking place under its "provisional" application.
"Some people believe that our verification activities are weak. this is not true," he said, citing the agency's modern system of remote monitoring alongside robust inspections.
And, far from being excluded from sites, IAEA inspectors are able to turn up -- in more than 90 percent of cases -- after giving the Iranians as little as two hours' notice.
Tero Varjoranta, Amano's deputy and head of the department of safeguards said: "Most of the access that we have done we have done clearly in less than 24 hours, in close to two hours."
Nevertheless, Amano remains concerned for the future if US hostility to the deal continues.
"The future of the JCPOA is uncertain, for now. The best thing is to fully implement the nuclear-related commitment, and we will do our very best to monitor and verify," he said.