Global powers involved in the Iran nuclear deal agreed Wednesday that the landmark accord was effective and that it should not be scrapped, said the European Union's foreign policy chief, Federica Mogherini.

"All parties are fulfilling the agreement," Mogherini told reporters at the United Nations General Assembly in New York, amid speculation that the US could be prepared to withdraw.

Although she could not guarantee that the US would remain part of the deal, Mogherini stressed that the EU was committed to preserving it.

Read more: Donald Trump and the Iran nuclear deal – a crisis in the making

Mogherini's comments came on the back of a meeting of top diplomats for the countries that negotiated and signed the Joint Comprehensive Plan of Action (JCPOA) with Iran in 2015 — China, France, Germany, Russia, the UK and the US.

The deal saw Iran agree to scale back its nuclear program in exchange for easing of sanctions and economic embargos.

In his speech at the UN General Assembly on Tuesday, US President Donald Trump called the Iran nuclear accord "an embarrassment" to the US.

Read more: What are Donald Trump's objections to the Iran nuclear deal?

Iranian President Hassan Rouhani rejected any renegotiation of the 2015 nuclear deal and said Tehran had "various" options" if President Trump decides to pull the United States out of the agreement.

"This agreement is not something you can touch. If you take out a single brick, the entire building will collapse," Rouhani told reporters after a speech to the UN General Assembly on Wednesday. "An American government that chooses to trample on her legal and legitimate international commitments, a conversation with such a government would be a waste of time."

Germany's leaders also made clear their opposition to any scrapping of the deal.

In an interview with DW, Ali Vaez, a senior Iran analyst at the Washington-based International Crisis Group (ICG), said killing the nuclear accord would force Tehran "to double down on its missile program and support for regional partners and proxies."

DW: The future of the Iran nuclear deal looks bleak. US President Trump threatened once again to pull out of the agreement with Tehran. Why does Trump so vehemently oppose the accord?

Ali Vaez: The Trump administration has no good reason to scuttle the Iran deal. If the US wants to strengthen the deal, it should supplement it with additional agreements that could serve the interests of both Washington and Tehran.

As French President Emmanuel Macron said, the fact that this deal was negotiated by Trump's predecessor [President Barack Obama] is not a good enough reason to scrap it.

Killing the deal is not going to moderate Iran's behavior in the Middle Eastern region. In fact, it will push Iran to double down on its missile program and support for regional partners and proxies.

It seems that Trump is not interested in diplomatic rapprochement with Iran as opposed to the Iranian government. Are you aware of any back door diplomatic efforts between the two countries to smooth out their differences?

Negotiations might only help if the Trump administration is seeking a better arrangement. Otherwise, no Iranian government will ever make additional concession to the US in return for nothing.

What is your impression of President Rouhani's General Assembly speech?

Rouhani had to deal with the situation cautiously in his speech. He had to appear moderate enough not to scare western investors and at the same time harsh enough not to be accused by his domestic critics of weakness in the face of Trump's confrontational policies. By that standard, his speech was a success.

Read more: Opinion: Iran's risky nuclear deal threat

What do you suggest to defuse the escalation between the US and Iran?

A stronger pact could not be built on the ruins of the Joint Comprehensive Plan of Action (JCPOA). The only solution is to preserve the deal and try to build on it.

Ali Vaez is a senior Iran analyst at the Washington-based International Crisis Group (ICG).

The interview was conducted by Shabnam von Hein.