EU foreign policy chiefs Monday reaffirmed their commitment to the Iran nuclear deal, in a pointed reminder to U.S. President-elect Donald Trump that he can’t renegotiate or walk away from the 2015 accord he has called “the worst deal ever negotiated,” “stupid” and “a disgrace.”

Trump has pledged to renegotiate the Joint Comprehensive Plan of Action, which was brokered by the United States, fellow permanent members of the U.N. Security Council China, France, Russia and the U.K., plus Germany (collectively known as the P5+1).

On Monday, EU foreign ministers meeting in Brussels underlined their support for the deal, citing four reports by the International Atomic Energy Agency suggesting that Iran is living up to its side of the agreement.

“The European Union reiterates the need for continued full and effective implementation of the JCPOA throughout the lifetime of the agreement,” the EU foreign ministers said in a joint statement.

One of the signature foreign policy achievements of Barack Obama’s presidency, it attempts to limit Iran’s ability to develop a nuclear weapon in exchange for an easing of international economic sanctions and Iran being allowed to develop civilian nuclear capabilities.

If the Trump administration tries to back out of it, the other parties will move ahead and ease sanctions provided Iran meets its commitments. That could lead to U.S. companies losing out on potential business deals as Iran is welcomed back into global markets and makes major purchases such as a new fleet of civilian airliners.

Some European nations have sent delegations to Iran in recent months in anticipation of renewed economic ties. EU foreign ministers said they were committed to resuming relations with Iran in the areas of  “political dialogue, human rights, economic cooperation, trade and investment, agriculture, transport, energy and climate change, civil nuclear cooperation, environment, civil protection, science, research and innovation, education, including through university exchanges, culture, drugs, migration, regional and humanitarian issues.”

Last week, Iranian Foreign Minister Javad Zarif warned Trump against trying to back out of the deal. “Every U.S. president has to understand the realities of today’s world. The most important thing is that the future U.S. president sticks to agreements, to engagements undertaken,” he said in Bucharest, where he led a delegation of 30 business leaders and diplomats.