The European Union's top diplomat said on Tuesday the bloc is working with Congress and the Trump administration to ensure America's new policy toward Iran doesn't lead it to violate a landmark nuclear deal.

Federica Mogherini said that in meetings in Washington this week she had stressed the importance the EU places on upholding the agreement. Under the 2015 accord, Iran curbed its nuclear program in exchange for an easing of international economic sanctions.

Mogherini allowed that it is “a delicate moment” for the deal since President Donald Trump refused last month to certify Iran's compliance, saying the benefits Tehran is getting from the deal are not proportional to its concessions. She said she is hopeful the US would continue to meet its obligations.

“We want to see the United States continue its implementation in the future,” she told reporters at a news conference. “The EU has a security interest in keeping the deal working.”

She declined to comment on whether legislation pending before Congress that would attach additional conditions for US sanctions relief would violate the deal, saying it was a domestic US political matter.

However, Mogherini said she had gotten a “clear indication that the intention is to keep the US compliant”. She did not elaborate.

The European diplomat's reticence to delve into the messy US political fight was echoed by Yukiya Amano, the International Atomic Energy Agency chief, who was in Washington meeting with US senators and top members of the Trump administration.

“This is a domestic US issue. We do not intervene,” said Amano, whose agency is charged with monitoring and verifying implementation of the nuclear deal. Amano said since the US remains a party of the agreement, his agency's work is unaffected by the political machinations in Washington.

Amano, in an Associated Press interview, said Iran's leaders had reiterated to him during his recent visit to Tehran that they remain committed to implementing the deal “under the existing framework”. He said the Iranians had pledged they won't be the first ones to “walk away” from the deal.

“They are watching very, very carefully,” he said.

Mogherini repeated the long-held EU position that the agreement cannot be reopened for negotiation, a stance also held by Iran and the other signatories.

“If you reopen one part of it, the entire agreement is reopened,” she said. “Renegotiating part of the agreement is not an option.”

Trump administration officials have said that in order for the US to remain in the agreement it must be fixed to remove so-called “sunset clauses” under which certain restrictions on Iran's activities expire. Deal opponents say those clauses provide a pathway for Iran to resume atomic development that could lead to the production of a bomb.

Mogherini said that despite the expiration of those limits, Iran is obligated under the agreement to never develop nuclear weapons.