Iranian President Hassan Rouhani told the UN General Assembly on Wednesday it would be a "great pity" if the landmark 2015 nuclear deal between Iran and world powers including the U.S. "were to be destroyed by rogue newcomers to the world of politics."

Rouhani spoke a day after U.S. President Donald Trump delivered an attack on Iran during his UN address in New York City, referring to the nuclear deal as an "embarrassment" and accusing Iran of exporting "violence, bloodshed and chaos."

Rouhani again vowed that Iran would not be the first country to violate the pact that saw Iran limit parts of its nuclear development program in exchange for significant concessions on economic sanctions. 

"I declare before you that the Islamic Republic of Iran will not be the first country to violate the agreement," Rouhani said. He added that Iran would respond "decisively and resolutely" if any party violates the accord.

"It will be a great pity if this agreement were to be destroyed by 'rogue' newcomers to the world of politics: the world will have lost a great opportunity," Rouhani said

Shortly before Rouhani's address, Trump told reporters he has made up his mind on the future of the nuclear deal, but refused to elaborate. 

"You'll see very soon," Trump said.

On Wednesday, the U.S. ambassador to the UN, Nikki Haley, said Trump's speech indicated his unhappiness but not a decision to abandon the accord.

"It's not a clear signal that he plans to withdraw. What it is, is a clear signal that he's not happy with the deal," Haley said in a CBS News interview.

Zarif's sharp words for Trump

The U.S. and Iran will have their highest-level interaction since Trump took office.

U.S. Secretary of State Rex Tillerson and Iranian Foreign Minister Mohammad Javad Zarif will attend a European Union-hosted meeting about the agreement.. The closed-door gathering is expected to be contentious, and the lead-up has seen Washington and Tehran trade increasingly harsh barbs.

A year ago, such a get-together would have been considered routine. In the current environment of animus, however, it is anything but.

Zarif, who had a friendly, collegial relationship with former secretary of state John Kerry while they negotiated the nuclear deal, was quick to denounce Trump's speech. He took to Twitter to offer a glimpse of what may be in store for future exchanges with U.S. officials, including perhaps with Tillerson on Wednesday.

"Trump's ignorant hate speech belongs in medieval times — not the 21st Century UN — unworthy of a reply," Zarif posted. "Fake empathy for Iranians fools no one."

With files from CBC News and Reuters