Federica Mogherini, the European Union’s foreign policy chief, visits the Capitol in an effort to defend the Iran nuclear deal. (Saul Loeb/AFP/Getty Images)

The Iran nuclear deal is working, according to two officials involved in implementing it who came to Washington on Tuesday to defend the embattled agreement before members of Congress.

Federica Mogherini, the European Union's foreign policy chief and coordinator of a joint committee charged with resolving disputes related to the nuclear deal, said this is a "delicate moment" for the 2015 agreement between Iran and six world powers, including the United States. Last month, President Trump refused to certify that Iran was complying with its commitments and instead tasked Congress with determining whether to take punitive action.

"We wish to see the United States continue in the implementation of the deal," Mogherini told reporters during a break between visits to Congress.

Separately, Yukiya Amano, head of the International Atomic Energy Agency, charged with monitoring and verifying Iran's compliance, rebutted criticism that Tehran may be engaged in prohibited activities at military sites where inspectors are denied access. While Amano declined to specifically address access at military locations, he said Iran has not blocked inspectors.

"When we need it, they have given us access," he said. Later, he added, "I can say we have had access to all the locations we needed to visit."

Mogherini and Amano came to Washington at a time when the Iran nuclear deal is under fire from the Trump administration and critics in Congress. Trump, who has called the pact — supported by the United Nations Security Council and the European Union — the "worst deal ever," has threatened to abandon it if Congress does not find a way to address his major criticisms.

Congress must decide by Dec. 12 whether to reimpose nuclear-related sanctions that were suspended when Iran agreed to curb its nuclear program. A U.S. withdrawal could either cause the deal to fall apart and allow Iran to resume its nuclear program, or isolate the United States further from the countries that also negotiated it and want it to remain in place as is: Britain, France, Germany, Russia and China.

"Renegotiation is not an option," Mogherini said, arguing that trying to renegotiate even one line in the 104-page agreement would open the entire deal for negotiations.

As Congress contemplates what actions to take, Mogherini said she is talking with lawmakers to ensure that any changes would not violate the existing agreement.

"I got clear indications that the intention is to keep the United States compliant with the agreement," she told reporters at a news conference.

Mogherini also pushed back against widespread criticism of the deal's "sunset provisions" of varying lengths, such as 10 or 15 years, after which Iran's commitments expire. She noted the permanence of the most significant provision.

"Article 3 says Iran will never develop a nuclear weapon," she said.

Amano said the Iran deal has caused the IAEA's workload to double in the past year, requiring more than 3,000 workdays for inspections, more than any other country. He said cameras are providing hundreds of thousands of images a day, and inspectors have installed 2,000 seals to render equipment inoperable. The collected data is the backbone of the IAEA's eight successive determinations that Iran is abiding by its commitments.

"We cannot provide 100 percent certainty," Amano acknowledged to a small group of reporters. "What we can provide is credible assurance."

Amano said that when he was in Iran last weekend, he met with government officials involved in the negotiations, including the president, the foreign minister and the head of the Iranian atomic energy commission. All assured him of Iran's commitment to the deal.

"They stated they would not be the first to walk away from the agreement," he said.

But he said the deal's fate is as unpredictable in Tehran as it is in Washington.

"They see uncertainties in the future of the JCPOA," he said, using an acronym for the agreement's official name, the Joint Comprehensive Plan of Action. "They stated they will continue to fully implement it. But the situation is not easy."